How much silica gel does my specimen box need?
Updated
On this page7
No conservation source publishes a grams-per-liter figure for a specimen box. The Canadian Conservation Institute (CCI), whose bulletin is the most detailed conservation source on silica gel, calls the old fixed amounts per volume "rather rigid" and replaced them with an equation in which the box's airtightness, the room's humidity, the humidity you want and how long the gel must last matter as much as the box's size [1]. Its equation needs an air exchange rate it gives only for display cases. So seal the box well, start with a layer of conditioned gel, and let a checked hygrometer inside tell you whether you have enough. Below: CCI's equation and sorbent table, and how to read the box.
What decides the amount
CCI's Technical Bulletin 33 (Tétreault and Bégin; first edition 2018, second edition 2020, revised 2023) gives the amount as an equation. It is reproduced here as reference, with the bulletin's own symbols and units [1]:
Q = (Ceq × D × V × N × t) ÷ (MH × F)
| Symbol | Meaning, as the bulletin defines it [1] | Unit |
|---|---|---|
| Q | "recommended quantity of dry sorbents" | kg |
| Ceq | "concentration of water vapour at equilibrium"; Ceq = 7.36 e^(T/19.96) - 2.75, which gives "17.3 g/m3 at 20°C" | g/m³ |
| D | "decimal difference between the RH outside the enclosure and the targeted RH inside" | none |
| V | "net volume of air in the enclosure" | m³ |
| N | "air exchange rate" | per day |
| t | "minimum number of days the targeted RH range must be maintained" | days |
| MH | "specific moisture reservoir of sorbent, including the effect of hysteresis" (from the sorbent table below) | g/kg for a 1% RH change |
| F | "targeted range of RH fluctuation" | % |
For N, the bulletin says "a value of one air exchange per day is commonly used to represent a typical, moderately sealed display case," and adds: "A well-designed glass and metal display case can easily reach an air exchange rate as low as 0.3 per day" [1]. It suggests a margin on top of the result: "As a safety factor, adding an extra amount of sorbent (such as 30%) is suggested" [1].
The bulletin's two worked examples
Both are for a display case of 1 m³ with N = 1, MH = 3 and F = 20 (a band of plus or minus 10 percent RH) [1]:
| Example | D | t | Calculation as printed | Q |
|---|---|---|---|---|
| Holding a target through a six-month cycle | 0.20 | 183 days | (17.3 × 0.20 × 1 × 1 × 183) ÷ (3 × 20) | 10.6 kg |
| Keeping a case drier than the room (50% outside, 20% inside) for a year | 0.30 | 365 days | (17.3 × 0.30 × 1 × 1 × 365) ÷ (3 × 20) | 31.6 kg |
Both results check: the arithmetic gives 10.55 kg and 31.57 kg.
What the equation means for a specimen box
Every term on the top line raises the amount and every term on the bottom lowers it. In practice:
| Factor | Needs more gel when | What you can do about it |
|---|---|---|
| Airtightness (N) | Air leaks in and out faster | Choose a better seal. Q rises in direct proportion to N, so a box that leaks a third as fast needs a third as much gel |
| Room against box (D) | The room is much damper (or drier) than the box should be | Store the box in the steadiest room you have |
| Allowed swing (F) | You allow only a small swing in RH | Accept the widest band your specimens tolerate |
| Time between recharges (t) | You want the gel to last longer | Recharge more often instead of adding more |
| Sorbent (MH) | The sorbent holds less water per percent of RH change | Pick a sorbent suited to the humidity you want (below) |
| Volume (V) | The box is larger | Use the smallest box that holds the specimens, the gel and a hygrometer |
As an illustration only: because Q scales with V, the first worked example's conditions in a 10 liter box (0.01 m³) would give about 106 g of dry sorbent, before the 30 percent margin. That figure holds only if the box leaks at one air exchange a day, and the bulletin gives no air exchange rate for household boxes. CCI's general page on humidity adds that in a leaky case adding silica gel "will make little difference" [2]. So treat any number you work out for a small box as a starting point to test with a hygrometer.
Pick the humidity before the gel
What the box should hold depends on the specimens. The bounds for a collection as a whole, 35 to 60 percent relative humidity (RH) from National Park Service guidance, live on humidity, high and low. A box exists for the pieces that need something different:
- Decaying pyrite and marcasite: as dry as you can get it. NPS says specimens with active decay need "a very dry environment" [3]; the Natural History Museum's Project Airless notes hydrates can form at RH "as low as 30%" once oxidation has begun [4]. See pyrite decay.
- Hydrated minerals that crack when dry (laumontite, borax, chalcanthite and others): held above 35 percent, often in a house that is too dry in winter. See dehydration and clouding.
- Carbonates with efflorescence: NPS guidance for Byne's disease says to keep RH "around 50% or lower" and names conditioned silica gel as one way to do it [5]. See efflorescence.
Never put a decaying sulfide and a hydrated mineral in the same box. They need opposite conditions.
Which sorbent for which job
CCI measured how much water each sorbent takes up or gives off per 1 percent change in RH, in three situations; this is the MH or M in the equation above [1]. Higher is better: more buffering from the same weight.
| Sorbent | MH, holding around 50% (plus or minus 10) | M, keeping RH low (20 to 30%) | M, keeping RH high (60 to 50%) |
|---|---|---|---|
| RHapid Gel | 6.11 ± 2.16 | 4.48 ± 0.42 | 4.38 ± 1.40 |
| 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 |
| Regular silica gel (clear, type A) | 1.93 ± 0.44 | 5.48 ± 0.40 | 1.47 ± 0.16 |
| 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 |
Values are grams of water per kilogram of dry sorbent for a 1 percent change in RH, at 20°C. MH includes the effect of hysteresis; M, in the other two columns, does not [1].
Two practical readings follow. Ordinary clear silica gel is the strongest of the group at keeping a box dry, so it is the natural choice for a sulfide or meteorite box. For a box held in the middle range, the buffered conservation products hold several times more water per percent of change than regular gel, so far less of them is needed. CCI measured these values on sorbents obtained from 2013 to 2015, and says: "New batches of these sorbents are not guaranteed to have the same performance" [1].
Conditioning, briefly
The bulletin's quantities are for dry sorbent, which then has to be brought to the humidity you want [1]. Methods for drying and moistening gel, with their temperatures and times, are on setting up a sealed low-humidity box. Three warnings from CCI belong here too [1]:
- Do not add liquid water to silica gel. CCI notes the many warnings against it in the literature, and in its own tests drops of water cracked every silica gel tested except Art-Sorb; it recommends slow absorption of water vapor as the safest method.
- Art-Sorb "cannot be heated in an oven above 65°C."
- Weighing tells you where the gel stands. CCI gives the water content of regular silica gel at 20°C: about 10.00 percent of its dry weight at 20 percent RH, 21.09 percent at 40 percent and 27.07 percent at 50 percent. Its example brings 1,000 g of dry gel to 1,270.7 g to condition it for 50 percent RH.
Finding the right amount by watching the box
Because the bulletin gives no air exchange rate for small boxes, the hygrometer does the measuring. A routine that follows the bulletin's logic:
- Test the seal first. Put dry gel and a hygrometer in the empty box. If the reading climbs back toward room humidity within days, fix or replace the box before adding specimens. Sealed-storage describes this check.
- Start with a layer and a margin. CCI's general page gives a starting design point for display cases: silica gel equal to about one tenth of the case height [2]. Use a layer of that depth under a perforated tray, then add the 30 percent margin the bulletin suggests [1].
- Check the hygrometer. CCI warns hygrometers can drift by about 5 percent RH and calls monitoring strongly recommended [1]. The salt check is on sealed-storage.
- Log readings. NPS asks for weekly readings and analysis where Byne's disease is a risk [5]; for a new box, weekly for the first month tells you how fast it drifts.
- Adjust. If the box reaches its limit before you want to recharge, add gel, recharge more often, or improve the seal. If it holds for months with no drift, you have enough.
- Keep it out of sun and away from heaters. CCI warns that "Rapid changes of more than 5°C in the enclosure may cause too large a disruption" in the gel's balance [1].
Record each setup and recharge in your catalog with the date, gel type and reading, so the history is there at the next annual condition check.
Building the box
The container, the gel tray, the hygrometer check and the oxygen-free option for decaying pyrite are all on setting up a sealed low-humidity box, and which plastics conservation sources consider stable for specimen contact is on which plastic is safe for specimens.
References
- Canadian Conservation Institute, Jean Tétreault and Paul Bégin, *Silica Gel: Passive Control of Relative Humidity*, Technical Bulletin 33, first edition 2018 (issued August 10, 2018), second edition 2020, revised 2023, page modified February 22, 2024, read October 22, 2026. https://www.canada.ca/en/conservation-institute/services/conservation-preservation-publications/technical-bulletins/silica-gel-relative-humidity.html
- Canadian Conservation Institute, *Agent of Deterioration: Incorrect Relative Humidity*, page dated February 5, 2021, read October 22, 2026. https://www.canada.ca/en/conservation-institute/services/agents-deterioration/humidity.html
- National Park Service, Kathy Hall, *Storage Concerns for Geological Collections*, Conserve O Gram 11/2, April 1998. https://www.nps.gov/subjects/museums/upload/11-02_508.pdf
- Kieran Miles, Matthew Porter and Amy Trafford, Natural History Museum, London, *Project Airless*, NatSCA blog, March 18, 2016. https://natsca.blog/2016/03/18/project-airless/
- National Park Service, Sally Y. Shelton, *Byne's "Disease": How To Recognize, Handle and Store Affected Shells and Related Collections*, Conserve O Gram 11/15, August 2008. https://nps.gov/subjects/museums/upload/11-15_508.pdf
