Conditioning Silica Gel

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Silica gel must be conditioned to the desired relative humidity before placement inside an exhibit case. Over time, the gel may require reconditioning.

What is meant by conditioning and reconditioning silica gel?

After the gel type and quantity are selected, the gel must be conditioned to the particular requirements of the case environment. To absorb moisture, thus drying out the case air, the gel may need to be adjusted down. To humidify or add moisture to the air, the silica gel may need to be adjusted up. Adjusting silica gel involves either removing or adding water to the gel in a controlled way. A slow but effective method used in many institutions over the years involves exposing the gel to the air of a controlled room, space or chamber which has the desired RH. In the field, silica gel moisture has even been removed by exposure to the sun on a dry day.

• Adjusting down: Although dry silica gel is assumed to be just that, it usually contains a small percentage of water. Silica gel used as a desiccant will need to be dried completely—as close as possible to zero humidity— by heating the gel in an oven, as described in more detail below.
• Adjusting up: Silica gel used in any capacity other than desiccation usually needs to be adjusted upwards. Moisture is added to the atmosphere of the dry gel until the desired moisture content of the case environment is achieved.

Is preconditioned silica gel available for purchase?

Most silica gel is shipped in a desiccated state. A few companies, however, do sell preconditioned silica gel ready for initial use. The manufacturer is told the percentage of humidity that is desired. Purchasing preconditioned silica gel is convenient, especially if you lack conditioning equipment, but is more expensive than buying unconditioned silica gel. In some instances, the gel can be sent back to the manufacturer for reconditioning.

How is the moisture content of the silica gel determined?

In order for the gel to be conditioned successfully, its moisture content must be monitored. This can be done in two ways: by weighing the gel or by monitoring the air surrounding the gel.
Weighing the gel: There are actually two different techniques that rely on the weight of the gel

1. The weight of a known quantity of silica gel relates to its moisture content. Many silica gel products come with a calibration chart giving weight relative to humidity level. If the gel does not come with a chart, you can create your own using the technique outlined below. Once the gel reaches the appropriate weight, the desired moisture content has been achieved.

Example: Expose a known quantity of silica gel (2 cups) in an environment of 20% relative humidity and record its weight. Slowly continue to raise the humidity and record the weight at each increase of 10%. (Note: Saturated salt solutions can be used) When the desired humidity level has been reached, record the weight on the chart. By doing this, you will have a record of the weights of the silica gel at different humidity levels. Because silica gels may vary among manufacturers, this calibration chart will be specific to the brand and type of gel used in the calculations.

1. If the gel is exposed to the desired relative humidity, it can be weighed until its weight stabilizes, indicating that it has reached an equilibrium state with the relative humidity in the control chamber. Weigh the gel every two to three days until its weight has stopped changing.

Monitoring the air: Perhaps the most direct way to determine the humidity level of the silica gel is to put a small amount of the gel (1/2 cup) in a small jar or zip-lock bag with a calibrated hygrometer (take care not to let the hygrometer directly touch the silica gel). After one to two hours the hygrometer should give an accurate reading of the gel's humidity level. This method can be used with any of the conditioning methods.

What methods can be used to condition silica gel?

Choose the method based on your equipment and the following considerations. A few general points apply to any of the techniques outlined below.

• The moisture content of the gel must be determined either through weighing the gel or by monitoring the relative humidity it maintains. (This can be done with a small sample of gel and is explained in more detail later in this TechNote.)
• The thinner the depth of silica gel, the faster the conditioning will take place; try to keep the depth below 1/2 inch. Periodic rotation of the gel to equalize moisture uptake is recommended.
• Use distilled water when introducing moisture.
• Once you believe that the desired relative humidity has been attained, store the gel in a sealed container and recheck the calibration the next day. The process may need to be repeated to ensure the desired moisture content.

SILICA GEL CONDITIONING AND RECONDITIONING TECHNIQUES

TECHNIQUE	DESCRIPTION	ADJUST DOWN	ADJUST UP	USE IN CASE	COST	RESPONSE TIME
Direct heat application	In a conventional oven spread loose gel in shallow, heat -resistant pan ½” deep; heat at 185°F for four hours (use lower heat for bags and cassettes). In a microwave oven set on high for two minutes; allow to cool for approximately 1 minute; and then repeat 10 times (or until dry).	yes	no	no	$$	rapid
Gradual exposure to new gel	Add a small quantity of conditioned gel (equal to 15-20% of the amount in the chamber or exhibit case) - saturated gel to raise moisture content or desiccate d gel to lower moisture content. Place the new gel near the existing silica gel in a separate container. Repeat daily until desired level is achieved.	yes	yes	yes	$	slow to variable
Room or chamber exposure	Place silica gel in a room or chamber which is at the desired level of relative humidity. The rate of conditioning can be increased by using a fa n to force air across the silica gel.	yes	yes	no	$$	slow to variable
Exposure to air conditioning unit	Place silica gel in a chamber attached to a microclimate generator set at the desired RH level. Conditioned air is blown into a chamber and over the gel. Most practical approach for large quantities of gel. Commercial units are available (see Suppliers). A mister or ultrasonic humidifier ca n be connected to the chamber as well.	yes	yes	yes	$$$$	rapid
Exposure to water vapor	Place a container of water within a chamber or case near the silica gel. Provide a large surface area of water to insure evaporation and to speed up the gel's absorption of water vapor. Placing wet sponges in the container is another method to speed up moisture release.	no	yes	yes	$	slow to variable
Saturated salt solutions	Enclose silica gel in a chamber with a saturated salt solution that maintains the desire d relative humidity. Do not use to condition silica gel inside a case (see TechNote on Silica Gel Conditioning)	no	yes	no	$$	variable

Products, Manufacturers, and Suppliers

Mention of a product, manufacturer, or supplier by name here is for information only and does not constitute an endorsement of that product or supplier. Listed materials have been used successfully in past applications. It is suggested that readers also seek alternate product and vendor information to assess the full range of available supplies and equipment.

Silica Gel Conditioning Chambers

APS Humidistat, Malone Displays, Decatur, GA, 30035

Automatic Reconditioning Unit, Art Preservation Services, New York, NY, 10128

Micro Climate Generator, Micro Climate Technology Inc., Burlington, CA, LZL584

References

LaFontaine, Raymond H, 1984. "Silica Gel", Technical Bulletin No. 10, Canadian Conservation Institute, Ottawa, pp. 4- 5.