Silica gel, commercial
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Description
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 1: 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 for | Recommended for |
|---|---|---|---|---|---|
| RH around 50% | Keep RH low | Keep RH high | Temp exhibit case in room with good to moderate climate control | Maintenance-free exhibit case | |
| RHapid Gel | 6.11 ± 2.16 | 4.48 ± 0.42 | 4.38 ± 1.40 | 2 kg/m3 | 4 kg/m3 |
| PROSorb | 5.42 ± 1.32 | 4.37 ± 0.10 | 4.25 ± 0.38 | ||
| Art-Sorb | 4.04 ± 0.80 | 2.84 ± 0.47 | 4.18 ± 0.49 | 4 kg/m3 | 8 kg/m3 |
| Arten Gel | 5.7 | 5.5 | |||
| Regular density silica gel (clear, type A) | 1.93 ± 0.44 | 5.48 ± 0.40 | 1.47 ± 0.16 | 9 kg/m3 | 18 kg/m3 |
| Orange silica gel | 1.16 ± 0.26 | 4.92 ± 0.59 | 0.94 ± 0.14 | ||
| 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 |
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. 32. 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.