Silica Gel Products and How to Use Them

Category:Exhibition
Category: Exhibit Technical Notes
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Moisture-absorbent products can be used in well-sealed exhibit cases to control relative humidity. Because silica gel acts both as a moisture absorber and desorber, it can buffer microclimates against excessively low or high RH levels-eliminating or reduce daily, weekly and seasonal fluctuation.
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==What is silica gel?== 
Silica gel is a hard, inert amorphous material available in granular, powder, or spherical bead forms. Commercial products are also available in which the gel is impregnated into other materials such as paper and foam polymers. Silica gel is non-combustible, non-corrosive, and non-toxic. Avoid breathing in the dust of silica gel, however, and remember that the impregnated products may be combustible or emit inappropriate volatiles.
There are two types of gels: standard silica gels and newer hybrid gels.
* Traditional silica gels are crystalline (2-5mm) and were developed for industrial products. The museum community has used this type of silica gel for decades, mainly as a desiccant to maintain lower relative humidities. (Eagle, Davidson and United Desiccant Gels are large suppliers of standard silica gel.)
* Hybrid gels were developed to be highly effective at both absorbing and desorbing moisture. Hybrid gels are better at maintaining mid-range relative humidities (40-70%) (ArtSorb, Arten Gel, Prosorb and Rhapid Gel are hybrid gels available commercially in several forms including loose beads, sheets, cassettes, and tiles).
==How does silica gel work?==
Silica gel can absorb up to 40% of its weight at high humidity levels. Its moisture absorbing capacity is based on large surface area created by surface pores and by interstitial micro pores that contain capillary channels. Polar water molecules drawn into the channels by capillary action are stored there until released. Silica gel can be used to maintain a specific relative humidity level (e.g., a low level as a desiccant) or to maintain an RH range (e.g., levels between 40 to 60% RH). Before use, the gel must be conditioned to the desired RH.
The effectiveness of the silica gel also depends on:
* a tightly-sealed exhibit case, with little or no air leakage to the ambient exhibit space;
* a maintenance compartment to store an adequate amount of gel, accessible for maintenance purposes; the gel can be hidden behind a panel in the bottom, ceiling, sides, or back of the case;
* air inside the case circulating over the silica gel;
* a method to contain the gel; silica gel should never touch collection objects, nor should you touch the gel with your bare hands;
* exposure of the gel over as wide a surface area as possible;
* temperature; silica gels absorb best at temperatures below 77° F (25° C).
==How do I choose a type of silica gel?==
Standard gel acts very well as a desiccant. Silica gel comes in many grades and mesh sizes. Indicating gel changes color from blue to pink when re-conditioning is required. In general, the indicating Grade 42 is very suitable for many museum applications. This can be mixed with the less expensive non-indicating Grade 40 which is approximately the same mesh size (6-16) or with a hybrid gel. Avoid the very fine powdered gels.
Hybrid gels, in general, are more expensive but also are more effective at maintaining a relative humidity in the range of 40-70%. Therefore, determine the target humidity range before choosing the type of gel to use. Hybrid gel products (such as ArtSorb, Arten Gel, Prosorb and Rhapid Gel) can be used in smaller quantities than the regular absorbency gels and come in a range of absorption properties. A disadvantage of the hybrid gels is that they cannot be purchased with a color indicator.
==How much silica gel should I use?==
Several factors will influence the amount of silica gel required for a specific exhibit application. Refer to the publication below to estimate the amount of moisture sorbent needed for your cases. Manufacturers of the gels could also be consulted regarding their respective products.
The amount of silica gel can be proportionately increased for the following mitigating factors:
* There is a very wide humidity differential between the case and the exhibit area.
* The RH fluctuation parameter to be maintained is extremely tight.
* The ambient temperature will fluctuate more than 20° F.
* The air volume of the case is very large relative to the size of the object(s).
* It is necessary to extend the time between silica gel maintenance (or eliminate the need altogether).
== Suggested Further Reading ==
Tétreault, J. and Bégin P. Silica Gel: Passive Control of Relative Humidity. CCI Technical Bulletin no. 32. Canadian Conservation Institute. Ottawa (2017). 
Thickett, D. "Specifying Air Exchange Rates of Showcases". Chemical Interactions Between Cultural Artefacts and Indoor Environment, Editor Mieke Adriaens, Susie Bioletti and Ira Rabin, ACCO, Leuven, Belgium ; The Hague, The Netherlands, 2018. pp. 25 – 48. 
Weintraub, S. "Demystifying Silica Gel". In Proceedings of the Objects Specialty Group Session, 30th AIC Annual Meeting, Miami, 10 June 2002: Objects Specialty Group Postprints, vol. 9. Washington, D.C.: American Institute for Conservation of Historic and Artistic Works, 2002, pp.169–194.
==Products, Manufacturers, and Suppliers==
Mention of a product, manufacturer, or supplier by name in this publication is for information only and does not constitute an endorsement of that product or supplier by the National Park Service. 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: Regular Density / Lower Absorbency Gel Fisher Scientific 
# Silica Gel: Blue Indication “tel-Tale” Indicating gel Conservation Materials
# Silica Gel: Hybrid High Absorbency Arten Gel Art Preservation ServicesArts-Sorb GelConservation Materials
# Silica Gel: Tiles and CassettesArt-Sorb Tiles and Cassettes Art Preservation Services Art-Sorb CassetteConservation Materials 
# Silica Gel: Embedded in SheetsArt-Sorb Polypropylene/polyethylene SheetConservation MaterialsRegular Density gel Embedded in Paper
Multiform Desiccant Products