Silica gel, commercial

==Description== 
An amorphous material of silica that has the ability to act as a humidity buffering agent. As a RH buffer, silica gel absorbs moisture in humid environments and releases water in dry air. Once equilibrated to set moisture level, silica gel will maintain that humidity level in a tightly sealed environment. Silica gel performs best at room temperature (70-80F) and high humidity (60-90%). The pore size in silica gel is related to its ability to adsorb water and other pollutants. Silica gel Type A with approximately pore size of 2.5 nm is most often used for vapor-phase moisture. Silica gel saturated with water can be dried with heat (120 C for 2 hours) and used again as a desiccant. For re-use as a humidity buffer, it must be reconditioned at the required humidity.
Product 	Composition 	Pre-conditioned RH options 	Humidity for usage 	Comments/Forms 	Web page for more info
Arten Gel 	type A SiO2 	0% 30% 40% 	Best for 0-40% range; avg value of MH* is 5 in range	8 mesh beads (2mm diameter) in moisture permeable bag 	APSNYC: Product page
Art Sorb 	90% SiO2 10% LiCl 	0% 40% 45% 50% 55% 60% 65% 70% 	Best from 40-70% 	Prosorb site says Art Sorb is best from 60-80%; beads, sheets, cassettes 	Fuji Silysia: Art Sorb
Moisture Pak 	cellulose, water, salts 	62% +/- 3% in sealed container 	Same 	Food storage, tobacco, wood instruments, herbs; variable size packets 	Sorbent Systems: MoisturePak
Prosorb 	97% SiO2 3% Al2O3 	30% 35% 40% 45% 50% 55% 60%	Optimum between 30-60% range 	Similar performance as Rhapid gel; beads, sachets, cassettes 	Sercalia: Prosorb
Rhapid gel 	Hybrid SiO2 	0% 40% 45% 50% 60% 	Best for 0-65% range; avg value of MH* is 5 in range	Flexible moisture-pearmeable sachets 	APSNYC: Products page
Silica gel E 	type A SiO2 	 	Best for 0-30% 	 	
Silica gel M 	Macroporous SiO2 	 	Best for 80-100% 	Prevents condensation at high RH 	
Silica gel orange	type A SiO2 with either methyl violet or iron salts as a moisture indicator	 	Best for 0-30% 	Moisture indicator: methyl violet changes from orange to green;iron salts change orange/brown to colorless;	AGM: Blue vs orange
Grade 42 	type A SiO2 with cobalt dichloride indicator 	 	 	Moisture indicator: changes blue to pink as RH increases from 15-45% 	
Grade 40 	type A SiO2 	 	 	may be mixed with 42 	
==Moisture Reservoir Information== 
The moisture adsorbing properties of silica gels are affected by factors such as capillary pore size or the inclusion of hygroscopic salts, resulting in a wide range of performance. Thus, it is important to compare the buffering capacity of different types of silica gels to determine which has the best performance for a specific application. In 1977, Thomson described the specific moisture reservoir with the variable M where M represents the amount of water (in grams) that is gained or lost by 1 kilogram of silica gel for each 1% change in RH. However for any specific silica gel type, M can vary widely due to the expected RH range and the hysteresis of the adsorption/desorption curve. In order to compensate, a new value called MH has been defined as the average amount of water (in grams) that is gained or lost by 1 kilogram of silica gel for each 1% change in RH. This is determined by repeatedly cycling silica gel between adsorption and desorption within a specific RH range until a constant value is measured. 
TABLE: Specific moisture reservoir (g/kg for a 1% RH change) at 20°C *
Moisture sorbent 	MH 50 ± 10% 	M 20 → 30% 	M 60 → 50% 	Recommended amounts
Moisture sorbent 	RH around 50%	Keep RH low 	Keep RH high 	Depends on air flow and exhibit length
RHapid Gel 	6.11 ± 2.16 	4.48 ± 0.42	4.38 ± 1.40	2-4 kg/m3
PROSorb 	5.42 ± 1.32 	4.37 ± 0.10	4.25 ± 0.38	2-4 kg/m3
Art-Sorb 	4.04 ± 0.80 	2.84 ± 0.47	4.18 ± 0.49	4-8 kg/m3
Arten Gel 	1.9 	5.5 	 	9-18 kg/m3
Regular density silica gel (clear, type A)	1.93 ± 0.44 	5.48 ± 0.40	1.47 ± 0.16	9-18 kg/m3
Orange silica gel 	1.16 ± 0.26 	4.92 ± 0.59	0.94 ± 0.14	9-18 kg/m3
Bentonite clay (Desi Pak) 	1.19 ± 0.07 	2.25 ± 0.03	1.11 ± 0.02	
Molecular sieves 4A (zeolite) 	0.33 ± 0.02 	0.47 ± 0.04	0.31 ± 0.01	
Table from: J. Tetreault (2017) with additional information from S. Weintraub (2002)
==Usage Calculators==
* Art Preservation Services: Silica Gel Recommendations 
* Small Corp: [www.smallcorp.com/silica-gel-calculator Silica Gel Calculator]
* Gaylord: [www.gaylord.com/resources/silica-gel-calculators Silica Gel Calculator]
== Resources and Citations ==
* AIC Conservation Wiki: Exhibit Technical Notes: Environmental Control
* Exhibit Guidelines Technical Note: Silica Gel Products and How to Use Them
* Exhibit Guidelines Technical Note: Conditioning Silica Gel
* Exhibit Guidelines Technical Note: Conditioning Silica Gel with a Saturated Salt Solution
* J.Tétreault, P.Bégin, Silica Gel: Passive Control of Relative Humidity, CCI Technical Bulletin no. 33. Canadian Conservation Institute. Ottawa (2017).
* Art Preservation Services: Silica Gel Technical Information
* S. Weintraub, "Demystifying silica gel", Objects Specialty Group Postprints, Vol.9, p. 169-194, 2002 Link
* G. Thomson, "Stabilization of RH in Exhibition Cases: Hygrometric Half-time", Studies in Conservation 22: 85-102, 1977. 
Category:Materials database Category:MWG Category: Climate/EnvironmentCategory: Comparisons