Desalination

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Contributors: Susanne Grieve, Laurie King, Julie Unruh

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Introduction[edit | edit source]

The chemical definition of a “salt” is a compound that contains an anion and a cation. One such compound is sodium chloride, or table salt, but there are many others. They include those found in groundwater and seawater, such as chlorides, carbonates, sulfates, phosphates, and nitrates; compounds in manufactured materials, such as phosphate fertilizers or chlorides-containing prepared food; and compounds that form as a result of contact with airborne organic acids, including sulfates, formates and acetates. The compounds can be categorized as “soluble” – easily dissolved in water – and “insoluble” – not actually insoluble, but less easily soluble. Soluble salt compounds cycle through solid/liquid phases with cycling relative humidity. In low humidity, soluble salts crystallize into solids; in high humidity, those solids dissolve.

Soluble salts may be introduced into porous materials in a number of ways, including via object use (for example, as a storage container for salted food); via conservation treatment utilizing materials such as plaster, acetic acid or hydrochloric acid; via off-gassing materials in a storage environment;[1][2] or, for archaeological material, via groundwater or seawater, since as water seeps into porous archaeological materials in burial, any salts compounds dissolved in the water are introduced into those materials.[3] In most terrestrial burial environments, relative humidity does not significantly fluctuate, and so the salts inside the object remain in a constant state. In marine deposition, because the object is immersed in water, the soluble salts remain dissolved. However, removed from the stable burial environment, a damaging cycle can begin.

Ceramic showing salt efflorescence. Retrieved from Ashmolean Museum, 23/03/2014.

In storage or exhibition environments, when the relative humidity is low, salts will crystallize - expanding many times in size, and exerting considerable pressures on the porous material. The pressures are great enough to cause structural damage.[3][4] When the relative humidity is high, the soluble salts again become liquid and mobile, and can travel via capillary action to new locations in the object, where new cycles of crystallization/dissolution will promote additional damage. Any porous material can be affected, including building materials such as mubrick, masonry, and wall plaster; and objects manufactured from materials including ceramic, stone, certain glass, bone, ivory, and organic materials.

If the salts species are chlorides, metals can also be affected. The presence of chloride salts can result in formation of chloride corrosion products that cause rapid structural damage to copper and iron alloys. In the case of archaeological material, the development of damaging corrosion may be inhibited during burial due to reduced oxygen, but once excavated, destructive corrosion species can develop quickly if associated chloride salts are not removed.

Soluble salts damage is irreversible. The goal of a desalination treatment is to remove soluble salts to prevent the possibility of that irreversible damage. However, desalination itself can also be destructive. Not only water-soluble salts, but any water-soluble component of the artifact can be altered or removed by desalination treatment, including water-soluble paint binders, coatings, and adhesives; organic elements of composite objects; some ceramic tempers; low-fired ceramics; and on a molecular level, residues that can clarify the object’s manufacture and use, biomarkers, and other compounds. Desalination treatment can therefore preclude certain types of analysis, organic residue analysis and DNA analysis in particular. It may be more important to preserve the artifact’s chemical integrity than its structural integrity. The advisability of treatment should always be evaluated case-by-case, and in consultation with researchers as necessary. It is rarely better to desalinate “just in case.” The likelihood of damage must be weighed against the potential destruction of information.

Predicating the likelihood of soluble salts damage is complex. In addition to artifact salinity, the degree of environmental RH fluctuations, salts species, the distribution of salts within the object, surface treatments, artifact thicknesses, artifact pore size, and, for ceramics, firing temperatures - among other factors - all affect whether damage will occur, where it will occur, and how much damage there will be [5][6][3]. In archaeological field conservation, there is a balance between maintaining an efficient artifact processing workflow on-site during an excavation season and mitigating the danger of deterioration and loss. Whether the risk of damage is great enough to justify delaying the workflow so that artifact desalination can be performed is site- and object-specific.

An alternative to desalination is environmental control. If relative humidity is stabilized, the salts will not cycle through destructive crystallization/mobilization phases, and damage will be prevented.[4] If relative humidity is kept low enough or if oxygen is removed, metals will not corrode. If environmental control is not possible storeroom-wide, microclimate storage containers can be constructed that will stabilize relative humidity or remove oxygen.

In general, archaeological objects from maritime sites have a higher risk of salts damage due to prolonged immersion in highly saline environments. Desalination of archaeological objects from maritime sites require special measures, discussed below.

Terrestrial site desalination[edit | edit source]

Ceramics desalination by immersion[edit | edit source]

Soluble salts activity in ceramics produces characteristic damage that includes flaking slips or glazes, microcracks that destabilize ceramic fabrics, and powdering, spalled, or lost surfaces. If desalination treatment is required, a standard desalination treatment involves soaking artifacts in water to remove enough of the salts so that remaining salts pose an acceptably low risk. (An alternative desalination treatment involves the use of poultices to remove soluble salts [7][8][9]). Though the actual kinetics are complex, the concept is simple: if immersed in a clean freshwater bath, solubilized salts will diffuse from the ceramic object into the water, at which point the water and salts can be discarded. It has been previously assumed that larger volumes of water would improve the efficiency and effectiveness of the treatment. It has been demonstrated that is not true in practice. [6][10] Similarly, it has been assumed that periodic water changes will increase salt removal: whether that is true has been shown to be species specific.[6] To date it is unverified that pure (deionized or distilled) water appreciably increases diffusion rate, another common assumption. Research is needed on this point, but there are indications that pure water may not be required to achieve artifact stability.[10] In most situations, it appears that a single static bath of relatively clean water that is large enough to completely cover the object is all that is needed, making it possible to successfully perform the treatment even in field locations with poor water resources.

Quantifying salts extraction[edit | edit source]

Measuring the conductivity of a water bath during the desalination process. Credit: INSTAP-Crete Study Center.

During desalination, extraction of salts can be quantified by measuring the salinity of the desalination bath with a conductivity meter either equipped with a species-specific electrode, such as a chlorides electrode, or an electrode that measures all dissolved salts. The basic principle behind a conductivity meter is that water that contains dissolved salts conducts electricity more efficiently than water that does not. A conductivity meter measures the time it takes an electrical current to travel between two electrodes: the shorter the time, the more dissolved salts in the solution.

Normalizing conductivity measurements[edit | edit source]

The volume of desalination bath water is a factor in the conductivity of the solution.

Conductivity measurements must be normalized with respect to bath water volume, mass of objects, and time in the bath[11] for reasons that are easy to understand intuitively. Imagine that 1 teaspoon of table salt is dissolved in 10 liters of distilled water. The resulting solution would not taste significantly salty – in fact the salt might not be detectable by taste at all. The conductivity of that solution would be very low. However, if the same 1 teaspoon is dissolved in only 10 ml of distilled water, the solution would taste horrendously salty, and a conductivity reading would be extremely high. Exactly the same quantity of salt is present in both cases: but without normalization, the conductivity measurements would indicate a low risk in the first case and a high risk in the second. In order to evaluate the actual degree of risk, the bath volume must be taken into account.

The mass of the ceramic affects how the amount of extracted salts should be interpreted.

Now imagine that two objects are desalinating in two baths of equal volume, and the conductivity measurements of both desalinations are the same. However, the first object is a large ceramic jar weighing 1000 gm, and the second object is a small ceramic flask weighing 100 gm. Even though the conductivity measurements indicate the same amount of extracted salts in both baths, the salts in the first bath came from a mass of ceramic that is 10 times greater than the mass of the ceramic in the second bath. To put that another way, 10 times more salts per gram of material were extracted from the small ceramic than were from the large ceramic. Even though the conductivity measurements of the two baths are identical, the small ceramic may be at risk while the large ceramic may not be. For that reason, the mass of ceramic material must also be taken into account.

Finally, imagine that all parameters are the same: two equally salty objects of equal mass are desalinated in baths of equal volume. In the first bath, the conductivity is measured after 5 minutes of immersion, and the conductivity is negligible. In the second bath, the conductivity is measured after 24 hours of immersion, and the conductivity is significant. Despite the difference in the conductivity measurements, the second object is not at greater risk - both objects began desalination with equal salt contents. The higher measurement in the second case is simply because more time has elapsed, allowing a greater quantity of salts to diffuse into the bath. For that reason, elapsed time between measurements must be taken into account.

Unlike direct measurements - the parameters of which may vary with every measurement, and so cannot be compared to other measurements - every normalized conductivity measurement can be directly compared to every other normalized measurement, regardless of the institution in which the treatment was performed, the artifact, artifact mass, the desalination bath size, or elapsed time. Because direct comparison of desalination curves for all objects is possible after normalization, an overall picture of the salinity of all ceramics with a particular storage history, all ceramics of one functional type, or all ceramics from from one site, stratigraphic level of the site, or feature can be compiled – even if those ceramics are in different collections. That data can in turn inform the development of treatment protocols for that group of material.

Normalized conductivity (knorm) is calculated as

knorm= ∆k*L / ∆t*g

Where k=conductivity in μS; L=volume of bath water in liters; g=weight of objects in grams; and t=time elapsed since previous measurement in days (or fraction of a day).[11] Because the math can be tedious, it can be helpful to record conductivity measurements in a spreadsheet that has been programmed to automatically calculate the normalized conductivity, such as an Excel spreadsheet or an OpenOffice spreadsheet. Doing so will also facilitate easy graphing, helpful for determining the endpoint of the treatment (see below).

Determining the endpoint[edit | edit source]

A typical desalination curve. In this case, the knorm reached a plateau in under 24 hours.

Early desalination protocols advised removal of all soluble salts. We now know that it is unlikely that all salts can actually be removed.[6] In consideration of damage that prolonged soaking may produce, updated protocols aim to desalinate for the shortest time necessary to lower the risk of soluble salts damage. At present there are several ways to calculate the endpoint of the treatment.[11][6][10] While additional research is critically needed to establish how much salt removal is necessary to mitigate risk, experientially, it has been reported that only removing that portion of salts that initially quickly diffuse into the bath can lower the risk to an acceptable point.[11][10] That being the case, a practical way to determine an endpoint is to establish the point at which the extraction rate stops changing significantly, or "plateaus." Graphing the desalination data may aid with that determination.

A numerical endpoint of knorm = 2 has additionally been suggested.[10] Based on experiential data only, that numerical endpoint appears to be valid in certain cases: additional research is still needed to determine whether that numerical endpoint is valid across a range of salt species mixtures, archaeological burial conditions, and other variables.

For ceramics that are not from marine archaeological contexts, there are indications that sufficient salts extraction may happen quickly – in some cases, in a matter of hours.[11] In the field, on archaeological sites that have a lower salt content, routine pottery washing may provide sufficient desalination.

Ceramics desalination by immersion: step-by-step[edit | edit source]

The following ceramics desalination procedure is adapted from White, Pool, and Carroll 2010[11] pages 48–49.

  1. Determine whether desalination by immersion is the best treatment for the situation. Consider the danger to any water soluble components and the impact on research value.
  2. Identify a desalination container that can accommodate the ceramic to be desalinated plus enough water to completely submerge the ceramic.
  3. Create a spreadsheet or table in which to record bath water volume, ceramic mass, date and time, and conductivity in µS.
  4. Measure and record the conductivity of the water to be used in the desalination bath.
  5. Record the dry ceramic mass in grams.
  6. Record the volume of water to be used in the bath in increments of liters.
  7. Completely submerge the ceramic in the bath water in the desalination container. The ceramic may be in the container as the water is added, or may be placed in the container after the full volume of water is added.
  8. Record immersion time to the minute.
  9. Observe the ceramic for a short period to verify that it remains stable underwater.
  10. Record an initial conductivity measurement within the first five minutes and as often as desired during the period of observation. For each measurement, record the time to the minute and the conductivity.
  11. For each conductivity measurement, calculate the knorm (or utilize a spreadsheet that calculates it for you]).
  12. Record additional conductivity measurements, the time taken, and the knorm at approximately one to five hour intervals. A set schedule is not necessary. Prior to measuring, gently stir the bath to distribute salts homogeneously.
  13. Remove the ceramic from the bath at the point at which the knorm values plateau, or alternatively, at knorm = 2. That point may be reached in a matter of hours; other desalinations may take several days.
  14. Remove any sediment on the surface of the object with clean water and a brush.
  15. Allow the ceramic to air dry.


Metals desalination[edit | edit source]

[Contributions sought for this section]

Marine site desalination[edit | edit source]

General[edit | edit source]

Desalination is essential to the long-term preservation of marine archaeological artifacts, as the objects will absorb soluble salts from the marine environment.[12] Soluble salts include chlorides, nitrates, phosphates, sulfates and carbonates, but chlorides are the most common catalyst of corrosion and damage to marine sites and artifacts.[13] There are a variety of methods available for desalination, depending on both the material components of the object and the capabilities of your facility.

A simple and convenient method for desalination is using a series of baths. Generally, the larger the volume of water in the desalination rinse, the more salts will be drawn out of the object per bath. Rinse baths should be conducted with deionized water, distilled water or tap water. Deionized water and distilled water are preferred as they inherently have a lower chloride content than tap water. However, tap water will have a lower chloride content than marine water, and will still remove some soluble salts if used for a rinse bath. If deionized and distilled water are limited, begin desalination with tap water and then do the final rinse baths in deionized or distilled water to remove the final remaining chlorides.[13][14][15]

Containers used for baths should be large enough to hold the object, or several objects if necessary. Containers should be water tight, and ideally have lids to reduce rinse water evaporation. Hard-top lids allow for stacking of smaller containers to save space.[15] Plastic sheeting or cling-film can be used in place of lids. If no lids or coverings are available, the desalination baths should be checked regularly as the rinse water may evaporate. During desalination, all objects should be completely submerged in rinse water. It has been found that stirring devices like pumps or water circulators can increase the rate of chloride removal. However, this can be a costly measure. Before adding water circulation, consider your timeline and the stability of objects to be desalinated. Friable or delicate objects should not be desalinated with water circulators.[14]

Factors such as the size and material of the object play into the amount of time an artifact will have to go through the desalination process. For example, the cannons brought up from the Beaufort Inlet Wreck, thought to be Blackbeard’s Queen Anne’s Revenge, are subject to years of the desalination process and will not be put on display until they are safely desalinated. Another example would be the artifacts recovered from USS Monitor which has conserved and displayed at The Mariners’ Museum and Park.[16]

Tracking and measuring desalination[edit | edit source]

In order to track the rate of desalination and estimate a desalination timeline, the amount of chlorides removed from the object into the rinsing bath must be measured and recorded. There are several methods to measure chloride removal.[4][17] Additionally, chloride testing interpretation can be dependent on the ratio of artifact surface area to the tank volume, the amount of chlorides present in the artifact, and the porosity of the object material.[14]

One method to monitor desalination is with a conductivity meter. Conductivity of the solution will increase as salts move from the object into the solution, and conductivity will plateau as salt diffusion slows.[18] Another accessible method is using chloride measuring strips such as Chloride QuanTab® Test Strips, which read chloride levels via titration in ppm, and come in both low range and high range options.[14][19] There are also methods using silver nitrate[20][12] and methods using analytical instruments, such as ion chromatography that measure chloride content.[21] The method best suited for your institution will depend on cost, convenience, and how often chloride content will be measured in the lab.

Example of a desalination graph, tracking the chloride content of a rinse solution for a marine archaeological object. When the chloride content plateaus, the rinse solution is changed until the chloride content is negligible. Courtesy of The Mariners’ Museum and Park, Newport News, VA.

Regardless of how chlorides are measured, the resulting readings should be tracked in a chart, with readings taken at regular intervals (i.e. daily or weekly). Testing is recorded as a graph of chlorides measured in parts per million (ppm) in the rinse solution plotted against time. By plotting regular chloride measurements, the rate of extraction can be determined, as can an estimated treatment endpoint. Typically, the chloride levels will rise until the rinse solution is saturated, and the chloride readings with plateau. At this point, the saturated solution should be disposed of, and the object submerged into a fresh solution. The chloride measurement process is continued until chloride levels drop to an acceptable range.[14] The acceptable range often varies slightly between institutions, depending on the risk of future corrosion and what chloride measurement tools are available. Some institutions desalinate until 25 ppm of chlorides is reached, others desalinate to 5 ppm.[22]

Desalinating organic materials[edit | edit source]

As stated above organic materials should be rinsed with a series of baths of deionized, distilled, or tap water. As waterlogged organics are generally fragile, no agitated desalination or water circulators should be used with organic desalination.[14] As organic materials are at risk for biological growth and decay, additional precautions should be taken. All organic desalination containers should be stored away from UV light to prevent bio growth. If possible, store the desalinating organic objects in cool storage (2-5°C or 35-40°F). If you do not have access to cool storage, simply store the objects in a naturally cool area away from natural light. Consider changing the rinse water more frequently to prevent biological growth.[15][23][17]

If a marine site and the objects recovered are large (small craft or ship timbers) adjustments will have to be made to desalinate large objects. Consider looking into other industries for large tanks; above ground pools or animal feed troughs can be successful desalination tanks. Alternatively, a series of pipes, hoses and sprinklers and soaker hoses can be used to rinse objects, though this will be less effective than soaking.[24][15]

Desalinating ceramics, glass and stone[edit | edit source]

Soluble salts can be damaging to ceramics glass and stone. If the object is dried before desalination, salts can crystallise within the pores, causing internal pressure and flaking.[15] Note that desalination is particularly critical for porous materials like terracotta, earthenware, sedimentary, and porous metamorphic rock, as they can more easily absorb salts from the environment. These materials may require a longer desalination process to remove all salts.[14][25] Glass in particular is at risk of delamination during desalination.[17]

Ceramics, glass and stone should be desalinated similarly to organic materials, placing the object into a clean, water-tight container and letting the object sit in a series of baths using deionized, distilled, or tap water. In the case of objects from low-chloride burial environments, it may be possible to simply change the rinse water daily, and measure chloride content at the end of each day or week to ensure sufficient salt removal. Rinsing in flowing water, for example in a sink, or with circulating water, is only recommended for more stable objects, as glass or ceramics may be at risk of delamination.[13][15]

Ceramics, glass and stone are also at risk of biological growth and decay, although the risk is lower than with organic materials. To prevent bio growth, keep desalination baths away from natural light. If biological growth occurs, change the rinse water more frequently to remove and prevent biological growth in the future.[17][15]

Desalinating metals[edit | edit source]

Desalination is absolutely vital for archaeological metal objects from marine sites. Chloride ions accumulate within archaeological metals during burial, and if not removed, can cause active corrosion, leading to spalling, flaking, weeping, and other damage.[12] The two most common methods for desalinating metal objects are chemical desalination and electrochemical desalination. Both techniques will be described below. Chemical desalination is conducted similarly to the desalination process described above for ceramics and organic objects, but with the use of an alkaline solution to aid in the removal of chlorides. The object is placed in a series of baths and chloride content is measured to track chloride extraction, until the desired chloride levels are reached.[26]

Pourbaix diagram illustrating the different regions the diagram can indicate. Tem5psu, CC By-SA 4.0, via Wikimedia Commons.

The object should be placed carefully in a water-tight container. Metal objects should not be desalinated in metal containers, as corrosion known as galvanic coupling can occur where the object touches the metal container. Instead, plastic containers or metal containers with a plastic liner are preferred.[14][12] The object should be submerged in the appropriate solution, generally an alkaline chemical dissolved in water. Deionized or distilled water are ideal. Different metals become stable at different pH levels, and so it is important to choose the correct alkaline solution for each metal. Pourbaix diagrams are visual representations of how a metal will theoretically behave under specific conditions, and are excellent tools to determine what pH is suitable for a particular metal during desalination.[27]

Pourbaix E(V/SHE)-pH diagram of Fe at 25°C. Diagonals correspond to the lower and upper limit of stability of the aqueous media. Metallos, CC BY-SA 3.0, via Wikimedia Commons.

As seen in the Pourbaix diagram to the left, passive conditions for iron are found in alkaline solutions. Because of this, iron artifacts are generally stored and desalinated in an alkaline solution, commonly 0.5M Sodium Hydroxide NaOH (2% NaOH by weight) in Deionized water, maintaining a pH of 12.[22][14][27] Solutions of 0.25M NaOH (1% NaOH by weight) or 1% KOH in Deionized water are also used.[26][27][28]

Pourbaix E(V/SHE)-pH diagram of Cu at 25°C. Metallos, CC By-SA 4.0, via Wikimedia Commons.

As seen in the Pourbaix diagram to the right, passive conditions for copper alloys are obtained at pH7 and at pH10. Tap water can be used to store and desalinate copper alloy artifacts at pH 7, or 0.5% Sodium Sesquicarbonate Na3H(CO3)2 (2% Na3H(CO3)2 by weight) in deionized water, which maintains a pH of 10.[22][14][15]

Two notes of caution. Firstly, both alkaline solutions mentioned above, 2% NaOH and 2% Na3H(CO3)2 in water are extremely caustic, due to the high pH, especially Sodium Hydroxide. If using these chemicals, proper personal protective equipment (PPE) should be used, including nitrile or chemical resistant gloves, apron, and eye protection. This is true both for when mixing the solution and when handling it in any way. Nitrile gloves at the minimum should be worn when sampling the solution to take chloride readings. Secondly, If alkaline solutions were used to adjust the pH of the desalination solution, you will need to check regulations in your area on how to properly dispose of the rinse solution after each bath.[22]

In order to ensure the removal of soluble salts, the solution must be measured for chloride content regularly. The same methods used for measuring chloride removal from organic objects can be used with metal objects and the alkaline solutions. Use one of the chloride measurement techniques mentioned above and record the data order time to track the rate of desalination.[14][22]

Desalinating metals: electrolytic reduction[edit | edit source]

Electrochemical desalination, generally known as electrolytic reduction, is commonly used on marine archaeological metals to remove chlorides. Electrolytic reduction is the production of a chemical reaction as a result of an electric current going through an electrochemical cell in which the artifact is the cathode.[12] Electrolytic reduction (ER) is used when there is a high chloride content and when the object is stable enough to undergo ER. To successfully conduct ER, the object must still have a metal core–this is especially key for cast iron objects.[14]

Illustration of common corrosion conditions of archaeological wrought iron objects. Illustration inspired by Cronyn, 1990, p.182.[4] Courtesy of The Mariners’ Museum and Park, Newport News, VA.

Cast iron contains a high amount of carbon, which increases the object’s brittleness. In marine cast iron objects, the high carbon content leads to a type of corrosion called graphitization, in which the outer layers corrode forming graphite. While the graphite will preserve the artifact’s original surface and any surface markings, it is extremely weak structurally.[17][14] Generally, marine cast iron objects maintain a metal core at the center of the object, which is surrounded by the graphitized outer surface. However, in some cases there will be no metal core remaining. If a cast iron object without a metal core undergoes electrolysis, the ER process will destroy the object. All cast iron objects must be x-rayed before ER to ensure there is a metal core.[22][12][17] If a cast iron object does not have a metal core remaining, the best course of action is to proceed with passive desalination, as described above. Iron objects must be analyzed by x-radiography before electrolytic reduction. X-ray analysis will not only indicate fractures and weak areas in any metal object, but it will indicate if iron objects have retained a metal core and give a general idea of how degraded an object is. Objects that retain a significant metal core can safely undergo electrolytic reduction. Those that do not have a metal core should only undergo passive desalination. If an iron object without a metal core were to undergo electrolytic reduction, this can lead to the complete destruction of the object itself.[4]

The X-ray of this cast iron flooring section shows which areas are weakest due to graphitization (darker section) and which areas still have significant iron core (white sections). Courtesy of The Mariners’ Museum and Park, Newport News, VA.

Wrought iron does not have the high carbon content of cast iron, and so does not have the same issues with graphitization. It is still suggested that wrought iron objects be x-rayed treatment, as the x-ray can indicate weak points in the material structure. X-radiography can also reveal the form of an object covered by marine concretion.[14]

Depiction of electrolytic reduction system desalinating an archaeological object. Image inspired by C. Volfovsky, p. 104.[29] Courtesy of The Mariners’ Museum and Park, Newport News VA.

Electrolytic reduction allows for the reduction of iron corrosion products, which in turn increases the removal rate of chlorides from the object into the desalination solution. In the case of ER, the desalination solution acts as an electrolyte, which allows ions (such as chlorides) to move freely. A direct current with negative charge is applied to the object, and the object itself acts as a cathode. A positively charged current is applied to a sacrificial anode. The negatively charged chloride ions are repelled by the negative current (the object) and the salt ions move into the desalination solution, increasing the rate of chloride removal.[22][14]

The catholic electric cable must have good contact with the metal object.[22] A metal clamp or alligator clip can be used to connect between the electric cable and the object. Any teeth or rough edges on the connector should be folded flat or removed to prevent damage to the object surface.[14] A multimeter is extremely helpful in confirming good conductive properties between two materials. Sacrificial anodes can be made of mild steel or stainless steel[22][30] The sacrificial anode should maintain a 20 cm to 80 cm distance from the object at all points.[22] The connection between the sacrificial anode and the electric cable should be raised above the level of the electrolyte solution for ER to function properly.[14]

The appropriate electrolyte to use in ER is determined by the material. The most common electrolyte used for iron and iron alloys is 2% NaOH in H2O, ideally deionized water. As with passive desalination 2% Na3H(CO3)2 in H2O, ideally deionized water is suitable for electrolytic reduction of copper alloy components.[22] The appropriate potential applied to the object is also dependent on the material type. The potential should be monitored regularly to prevent deviation outside of the desired range.[27] The use of reference electrodes is recommended as reference electrodes provide a more accurate reflection of the potential in the object itself.[26]

As with passive desalination, the object and the entire ER system should be placed in a water-tight, non-metal container. The non-metal container is doubly important during ER, as a metal container could interfere with the reduction reaction and the removal of chlorides. The entire ER system should be submerged in the desalination solution, and the solution should be sampled regularly for chloride measurement, so that the rate of chloride extraction can be monitored. Additionally, caution should be taken when using alkaline solutions for desalination, as mentioned above.[14][12][22]

Rinsing[edit | edit source]

Following desalination with an alkaline solution, such as 2% NaOH or 2% Na3H(CO3)2 the object must be rinsed to remove excess chemicals. The object should be placed in a bath of deionized water. The rinse bath should be changed daily, and the pH of the rinse bath should be monitored, as the residual NaOH or Na3H(CO3)2 on the object surface will raise the pH of the solution. When the solution reads pH 7, the excess chemicals have been fully rinsed and the object can move to drying. As a note, this rinsing process will likely induce flash corrosion on the surface of iron objects. The best solution for this is that before the solution is changed daily, brush the surface of the object with a natural or synthetic hair brush, like a toothbrush. This will remove flash corrosion from the surface, and the solution change will aid in rinsing the flash corrosion away. There has been some success in using sodium nitrite as a corrosion inhibitor during the rinsing process with iron objects from USS Monitor.[31]

Dehydration[edit | edit source]

All marine archaeological objects should be dehydrated or dried following desalination. The drying method and any required associated steps, like consolidation or bulking, depend on the material type.

Organics[edit | edit source]

Many waterlogged organics require consolidation or bulking after desalination and before drying.[32] The best consolidant is dependant on the object material, though polyethylene glycols (PEG) are a common choice for many organic materials.[14][24][22] In some cases, consolidant choice will be dependant on the type of wood, or type of leather, etc.[15][33]

In the case of wood, if the object is large, as with ships and boats, the consolidant and drying method should be based on the scale of the project, the facilities available, and the health and safety requirements for the staff.[24][32][15] Waterlogged archaeological wood, leather, textiles, cordage, matting and basketry generally respond best to controlled air drying or freeze drying when pre-treated through consolidation or bulking.[22] Without a bulking treatment, organics are at risk of shrinking, warping, and loss of structural strength during the drying process.[24][32] Bone, antler, and ivory all have different densities and can dry at different rates. Identify which materials are in your collection so each be conserved and dried in the most appropriate method.[34]

Vacuum freeze drying[edit | edit source]

Vacuum freeze-drying removes moisture from objects through sublimation, which is generally less damaging to organic materials drying at room temperature. Archaeological waterlogged wood, leather, textiles,cordage, matting and basketry have all been successfully dehydrated with vacuum freeze drying, usually with the use of a pre-drying consolidant or bulking agent.[22][32]

A typical weight loss curve during the freeze drying process. Courtesy of The Mariners’ Museum and Park, Newport News VA.

When vacuum freeze drying organic objects, the object should be placed on its support as it will be displayed or stored. This may be flat or with a 3-D support which molds to the object's original shape.[22][14] The object and its support are then placed in the freezer for a minimum of 24 hours to reach freezing temperature. The object is then placed with support into the freeze dryer, once the freeze dryer has come to temperature. During the drying process, remove the object from the freezer every other day and weigh the object. Track the weight on a graph plotting the weight of the object vs time. The object should lose weight as the water sublimates from the object. When the object weight plateaus, the object has finished drying.[24][35]

For specifics on the freeze drying process, how freeze-dryers function, and the required temperatures and atmospheric pressure to induce freeze-drying, turn to the Canadian Conservation Institute (CCI) Notes 4/2.[35]

Some facilities do have vacuum freeze dryers large enough to hold entire timbers and small watercraft. For example, the National Museum of Denmark was able to vacuum freeze dry Roskilde 6, an archaeological warship dating to around 1025.[24]

Non-vacuum freeze drying[edit | edit source]

Inherently fragile materials like textiles are often suited for dehydration via non vacuum freeze drying. It allows for a simple and effective drying process with minimal risk of damage to the object.[14] There has also been success with non vacuum freeze drying leather, bone, wood, and keratinous materials.[22]

The object should be placed on its support as it will be displayed or stored. The object and its support are then placed in the freezer. Within 6 weeks water will sublimate from the object. To track the drying rate, weigh the object (and support if it will be dried with the support) before placing it into the freezer.[14] During the drying process, remove the object from the freezer every other day and weigh the object. Track the weight on a graph plotting the weight of the object vs time. The object should lose weight as the water sublimates from the object. When the object weight plateaus, the object has finished drying.[24]

If the freezer is a humid environment, consider creating a micro-environment and using silica gel or other methods to remove excess moisture. Check to object at regular intervals to ensure mold or other biological growth is not developing as the object dries.[14]

Controlled air drying[edit | edit source]

Rubber has been found to have successful results with controlled air drying, creating a microenvironment to lower the humidity at room temperature.[36] When pre-treated with a bulking agent, leather responds well to controlled air drying. Archaeological textiles can be air dried both with or without a consolidant; the need for a consolidant depends on the textile material and object stability.[22][15] Bone, antler, ivory, and keratinous materials show success with controlled air drying. For stable objects, laying the object on an open mesh rack and allowing it to evenly air dry across the surface will be sufficient. For more fragile objects, a slower drying process using a controlled micro-environment may be more successful. During the drying process of any organic materials, mold and biological growth is a risk, so objects must be checked thoroughly and often for any signs of mold growth.[34][15]

For an example of consolidation and controlled air drying on a large-scale, turn to the Vasa. The Vasa is a battleship from 1628, which was recovered in 1961. The ship was sprayed with PEG as a bulking method. Over several years, the relative humidity was decreased from 95% to 58% by conservators.[24][32]

Ceramics, stone, glass[edit | edit source]

Controlled air drying[edit | edit source]

Following desalination, ceramics, stone, and glass can be dehydrated via air drying. In most cases the objects can simply be removed from the rinse and laid out in a well ventilated area to dry. Ideally, the objects should be placed on a stable surface, with clean paper or cloth towels to absorb excess moisture. Any unstable objects can be supported with ethafoam or similar material during drying.[14]

Solvent drying[edit | edit source]

Particularly delicate objects can be dried using solvent drying if necessary. Solvent drying will lessen the surface tension of effect of water and minimize surface damage, so this is suggested for objects that are friable or at risk for delamination. The object should be placed in two or three successive baths of acetone or denatured alcohol, with each bath lasting one hour.[14]

Metals[edit | edit source]

Oven drying[edit | edit source]

Wrought iron objects can be dried via oven drying. This is a fairly accessible method for drying, but please note that most cast iron objects are not stable enough to undergo oven drying, and the graphitized surface layer could flake off of the object during the drying process. While wrought iron can safely undergo oven drying, some flask corrosion will occur on the surface. This can usually be brushed off using simple mechanical cleaning used with iron objects.[14]

To oven dry wrought iron objects, preheat the oven to 350 degrees F or 177 degrees C. For ease of transportation and even support of the objects, place the objects on an oven-safe tray. From the objects from their rinse bath and place the tray in the oven for 24 to 48 hours. The length of time can vary depending on the size of the object. After drying is complete, allow the object to cool to room temperature. And flash corrosion can be removed with mechanical cleaning.[14] Keep the object in low humidity (less than 30%) storage until the object is ready to be coated.[17]

Copper alloys can be dehydrated using the oven drying process, using the same method as with wrought iron. However, oven drying can darken and discolor copper and copper alloys. If possible, solvent drying is the preferred method of drying copper and copper alloys.[14]

Solvent drying[edit | edit source]

Any metal (except tin and aluminum) can be dehydrated with solvent drying. However, it is a more delicate process, a more expensive process, and will also require more health and safety protocol. Ensure you have a well ventilated area to conduct solvent drying or more preferably, a fume hood. Familiarize yourself with the SDS associated with the solvent used, and ensure you have all required PPE and a solvent disposal plan.[14] Ensure all solvent containers (including baths) are clearly marked, water tight, and have air-tight lids. The use of air-tight lids will both minimize solvent evaporation and limit your exposure to organic solvents.

Solvent drying involved submerging waterlogged objects into three successive baths of acetone or denatured alcohol. The object should be submerged in the bath for at least one hour before moving to the next, and larger objects should be submerged longer. After the object has been submerged in three baths for one hour each, The object can be set in the open air, under ventilation, which will evaporate any solvent remaining on the surface and dry the object.[14] Keep the object in low humidity (less than 30%) storage until the object is ready to be coated.[17]

Silica gel drying[edit | edit source]

In addition to using silica gel to maintain a low humidity environment, silica gel can be used to dry waterlogged metal artifacts. This should be conducted in an air-tight container larger than the object itself. Silica gel can be placed in the container with the object, but not touching the object. Humidity in the enclosed environment should be monitored with a hygrometer or humidity card. The silica gel will likely need to be changed regularly to maintain a low humidity environment.[37][38] An enclosure can be made for larger objects by simply using plastic sheeting to make a custom container, which can be closed with tape or clamps.

References cited[edit | edit source]

  1. ↑ Paterakis, A.B., and M. Steiger. 2015. “Salt Efflorescence on Pottery in the Athenian Agora: A Closer Look.” Studies in Conservation 60 (3) (May):172–84.
  2. ↑ Halsberghe, L., D. Erhardt, Gibson, Lorraine T, and Zehnder, Konrad. 2005. “Simple Methods for the Identification of Acetate Salts on Museum Objects.” In Preprints of the 14th Triennial Meeting, The Hague, 12-16 September 2005, 2:639–47. James & James.
  3. ↑ 3.0 3.1 3.2 Charola, A. Elena. 2000. “Salts in the Deterioration of Porous Materials: An Overview.” Journal of the American Institute for Conservation 39 (3): 327–43. https://doi.org/10.1179/019713600806113176.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 Cronyn, J. M. 1990. The Elements of Archaeological Conservation. Routledge.
  5. ↑ Santarelli, Brunella, and Nancy Odegaard. 2012. “Salt Damage Related to Material Properties of Ceramics.” Poster presented at the AIC Annual Meeting. https://www.culturalheritage.org/docs/default-source/publications/annualmeeting/2012-posters/35-saltdamagerelated.pdf?sfvrsn=ef13338d_8.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 Charola, A. Elena, J. Freedland, and Silvia A. Centeno. 2001. “Salze in Poröser Keramik IV: Überlegungen Zum Entsalzen / Salts in Ceramic Bodies IV: Considerations on Desalination.” Internationale Zeitschrift Für Bauinstandsetzen Und Baudenkmalpflege 7 (2): 161–74. https://doi.org/10.1515/rbm-2001-5555.
  7. ↑ Dinneen, Brittany Dolph, Jessica Betz Abel, and Renée Stein. 2024. “Using a PH-Adjusted Semi-Rigid Agarose Gel with Ion Exchange Resin for Poultice Desalination: Preliminary Experimentation and Case Studies.” Journal of the American Institute for Conservation 63 (4): 309–23. https://doi.org/10.1080/01971360.2024.2361547.
  8. ↑ Verges-Belmin, V., and H. Siedel. 2005. “Desalination of Masonries and Monumental Sculptures by Poulticing: A Review / Entsalzen von Mauerwerk Und Steinfiguren Mit Hilfe von Kompressen: Ein Überblick.” Restoration of Buildings and Monuments 11 (6): 391–408.
  9. ↑ Lee, Lai-Mei, Philip Rogers, Victoria Oakley, and Juanita Navarro. 1997. “Investigations into the Use of Laponite as a Poulticing Material in Ceramics Conservation.” Conservation Journal January 1997 (22).
  10. ↑ 10.0 10.1 10.2 10.3 10.4 Unruh, Julie. 2001. “A Revised Endpoint for Ceramics Desalination at the Archaeological Site of Gordion, Turkey.” Studies in Conservation 46 (2): 81–92. https://doi.org/10.1080/00393630.2001.12071696.
  11. ↑ 11.0 11.1 11.2 11.3 11.4 11.5 White, Chris, Marilen Pool, and Norine Carroll. 2010. “A Revised Method to Calculate Desalination Rates and Improve Data Resolution.” Journal of the American Institute for Conservation 49:45–52.
  12. ↑ 12.0 12.1 12.2 12.3 12.4 12.5 12.6 Selwyn, Lyndsie. 2004. Metals and Corrosion: A Handbook for the Conservation Professional. Canadian Heritage. Canadian Conservation Institute. Ottawa.
  13. ↑ 13.0 13.1 13.2 Buys, Susan and Victoria Oakley. 1993. The Conservation and Restoration of Ceramics. Routledge. New York.
  14. ↑ 14.00 14.01 14.02 14.03 14.04 14.05 14.06 14.07 14.08 14.09 14.10 14.11 14.12 14.13 14.14 14.15 14.16 14.17 14.18 14.19 14.20 14.21 14.22 14.23 14.24 14.25 14.26 14.27 14.28 14.29 Rodgers, B. 2004. The Archaeologist's Manual for Conservation. Springer, New York, NY.
  15. ↑ 15.00 15.01 15.02 15.03 15.04 15.05 15.06 15.07 15.08 15.09 15.10 15.11 Jones, M. (Ed.) 2003. For Future Generations: Conservation of a Tudor Maritime Collection. Oxbow Books, Philadelphia.
  16. ↑ Hoffman, W., Sangouard, E., Farrel, E., Haines, L., King, L., McGath, M. 2022. Building an Ironclad System: A Quarter-Century of Innovation in the Conservation of USS Monitor. Paper presented at the ICOM-CC Metal Specialty Group Conference in Helsinki, Finland.
  17. ↑ 17.0 17.1 17.2 17.3 17.4 17.5 17.6 17.7 Robinson, Wendy. 1998. First Aid for Underwater Finds. Archetype Publication, Nautical Archaeology Society, London, Portsmouth.
  18. ↑ Panther, I., Crawshaw, A. 2023 Use of a conductivity meter to monitor desalination. Wet Organic Archaeological Materials 2023. Proceedings of the 15th ICOM-CC Wet Organic Archaeological Materials Working Group Interim Meeting, Mainz.
  19. ↑ Selwyn, Lyndsie. 2016. Test for Chloride Ions in Iron Treatment Solutions Using Quantab Test Strips – Canadian Conservation Institute (CCI) Notes 4/4. https://www.canada.ca/en/conservation-institute/services/conservation-preservation-publications/canadian-conservation-institute-notes/test-chloride-ions-iron-treatment.html
  20. ↑ Selwyn, Lyndsie. 2016. How to Test for Chloride Ions in Iron Treatment Solutions Using Silver Nitrate – Canadian Conservation Institute (CCI) Notes 4/5. https://www.canada.ca/en/conservation-institute/services/conservation-preservation-publications/canadian-conservation-institute-notes/test-chloride-ions-iron-treatment-silver-nitrate.html
  21. ↑ Willey, Jo. 1995. The Effects of Desalination on Archaeological Ceramics from the Casas Grandes Region in Northern Mexico. MRS Proceedings:352:839. DOI: 10.1557/PROC-352-839
  22. ↑ 22.00 22.01 22.02 22.03 22.04 22.05 22.06 22.07 22.08 22.09 22.10 22.11 22.12 22.13 22.14 22.15 22.16 22.17 Pearson, C. Ed. 1987. Conservation of Marine Archaeological Objects. Butterworths, Boston.
  23. ↑ Montembault, V. and B. Wills (eds.). 2001. Treatments of Archaeological Leather in France. In Leather Wet and Dry: Current Treatments in the Conservation of Waterlogged and Desiccated Archaeological Leather. Archetype Publications for The Archaeological Leather Group. London:45-50.
  24. ↑ 24.0 24.1 24.2 24.3 24.4 24.5 24.6 24.7 Hoffmann, Per. 2013. Conservation of Archaeological Ships and Boats. Archetype Publications, London.
  25. ↑ Logan, Judy, and Charlotte Newton. 2007. Care of Ceramics and Glass – Canadian Conservation Institute (CCI) Notes 5/1. https://www.canada.ca/en/conservation-institute/services/conservation-preservation-publications/canadian-conservation-institute-notes/care-ceramics-glass.html
  26. ↑ 26.0 26.1 26.2 Guilminot, E., Neff, D., Remazeilles, C., Reguer, S., Kergourlay, F., Pele, C., Dillmann, P., Refair, P., Nicot, F., Mielcarek, F., Huet, N., Rebiere, J. 2012. Influence of crucial parameters on the dechlorination treatments of ferrous objects from seawater. Studies in Conservation 57(4):227-236.
  27. ↑ 27.0 27.1 27.2 27.3 Degrigny, C. 2010. Use of electrochemical techniques for the conservation of metal artefacts: a review. Journal of Solid State Electrochemistry 12:353-361.
  28. ↑ Kergourlay, F., Guilminot, E., Neff, D., Remazeilles, C., Reguer, S., Refair, P., Mirambet, F., Foy, E., Dillmann, P. 2010. Influence of corrosion products nature on dechlorination treatment: case of wrought iron archaeological ingots stored 2 years in air before NaOH treatment. Corrosion Engineering, Science and Technology 25(5):407-413.
  29. ↑ Volfovsky, C. 1999. Chimie - Electrochimie 1 - Nettoyage - dechloruration - conservation. In A la recherche du métal perdu, Catalogue d’exposition du Musée archéologique du Val d’Oise, ed. H. Meyer-Roudet, Saint-Germain-du-Puy: France:98-113
  30. ↑ Hamilton, D. L. 1996. Basic Methods of Conserving Underwater Archaeological Material Culture. U.S. Department of Defense, Legacy Resource Management Program. Washington D.C.
  31. ↑ Sangouard, E., Nordgren, E., Spohn, R., Brunke, K., Krop, D. 2015. Evaluation of sodium nitrite as a corrosion inhibitor for USS Monitor artifacts. Studies in Conservation 60(4):253-266.
  32. ↑ 32.0 32.1 32.2 32.3 32.4 Rowell, R.M. and R. J. Barbour, eds. 1990. Archaeological Wood Properties, Chemistry, and Preservation. American Chemical Society, Washington, DC
  33. ↑ Peacock, E.E. 2001. Water-degraded Archaeological Leather: An Overview of Treatments Used at Vitenskapsmuseum (Trondheim). Leather Wet and Dry Current Treatments in the Conservation of Waterlogged and Desiccated Archaeological Leather (B. Wills, ed.). Archetype Publications for The Archaeological Leather Group, London:11-25.
  34. ↑ 34.0 34.1 Grant, Tara. 2007. Conservation of Wet Faunal Remains: Bone, Antler and Ivory – Canadian Conservation Institute (CCI) Notes 4/3. https://www.canada.ca/en/conservation-institute/services/conservation-preservation-publications/canadian-conservation-institute-notes/conservation-faunal-bone-antler-ivory.html
  35. ↑ 35.0 35.1 Cook, Clifford. 2007. Vacuum Freeze-drying Archaeological Artifacts – Canadian Conservation Institute (CCI) Notes 4/2. https://www.canada.ca/en/conservation-institute/services/conservation-preservation-publications/canadian-conservation-institute-notes/vacuum-freeze-drying-archaeological-artifacts.html
  36. ↑ King, L., Haines, L., McGath, M. 2020. A Sticky Situation: Conservation of Historical Waterlogged Rubber. Poster presented at the 48th Annual Meeting of the American Institute for Conservation Conference (Virtual).
  37. ↑ Cook, Cliff. 2019. Preparing Silica Gel for Contained Storage of Metal Objects – Canadian Conservation Institute (CCI) Notes 9/14. https://www.canada.ca/en/conservation-institute/services/conservation-preservation-publications/canadian-conservation-institute-notes/prep-silica-gel.html
  38. ↑ Logan, Judy, rev. Lyndsie Selwyn. 2007. Storage of Metals – Canadian Conservation Institute (CCI) Notes 9/2. https://www.canada.ca/en/conservation-institute/services/conservation-preservation-publications/canadian-conservation-institute-notes/storage-metals.html


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