Last changed: 6/2009

topHow much PROSORB do you need for your specific showcase?

Unfortunately, it is quite hard to predict the necessary amount of humidity buffer in a given situation. It depends of several factors. Some of them might be known, but generally not all of them. The necessary amount depends on:

As a consequence, one always has to start with a rough guess of the necessary quantity (for example 2 – 4 kg / m³) and monitor RH permanently with a hygrometer. It’s good to be able to increase the quantity of buffer in case the desired RH level is not reached, or service intervals are considered too short. Therefore the space for the buffer should preferably be large with some “safety margin”.

topCase design for passive climate control with PROSORB

It doesn't matter where you put the PROSORB inside your showcase as long as the following conditions are fulfilled:

Cassettes should be placed horizontally only. Sachets are dustproof and also can be turned upside down, hung vertically or horizontally, etc.

topRe-conditioning of PROSORB

PROSORB is delivered to you conditioned to a given RH, with a tolerance of ±2% RH. These values refer to 20°C. Cassettes are labeled with the gross weight at a given RH, sachets have their gross weight stamped on the top.
The following instructions describe how to re-condition a PROSORB cassette or a sachet to the desired weight (i.e. the weight that is printed on the cassette or the sachet). If you want to condition the cassettes to a different RH, you can estimate the corresponding gross weight by using the adsorption values given in Table 1. But this estimation will only be approximate and should always be cross-checked with a calibrated hygrometer.

Please note: all types of silica gel will lose some of their RH-regulating properties over time. Therefore the conditioning of a PROSORB cassette needs to be checked with a calibrated hygrometer at regular intervals. Initially this should be done after two years, and thereafter once a year. Depending on the measurement results, the weight of the cassette will need to be adjusted.

Humidification of PROSORB cassettes or sachets (increasing their weight):

Please note that PROSORB-beads will crack in direct contact with liquid water. Therefore all humidification must take place in the gas phase. You can humidify the cassettes by exposing them in a humid room or in a humidification chamber until they have reached the correct weight.
But you also can humidify the cassettes in polyethylene-bags:

  1. Weigh the cassette in order to determine how much its weight needs to be increased.
  2. Take some layers of tissue paper, blotting paper or cloth and pour the calculated quantity of water onto this material. The water has to be absorbed completely without dripping at the edges.
  3. Place this humidified paper or cloth on top of the cassette and place it in a polyethylene bag.
  4. Securely close the bag and wait until the water is completely absorbed by the cassette (several days).
  5. Weigh the cassette to make sure that it has reached the desired weight.
  6. As you can imagine, the upper layers of PRO SORB will be much wetter now than the beads on the bottom of the cassettes. So mix the beads well by slowly turning the cassettes - or let the cassette equilibrate for some days.
  7. Measure the RH by putting the cassette and a calibrated hygrometer in a small enclosure such as a polyethylene bag, securely close the bag and after approximately two hours read the RH measurement (see photo).

Drying PROSORB cassettes (reducing their weight):

There are a number of ways in which a cassette can be dried and its weight reduced. Three of them are described below (Instructions A, B or C):

A - Heating the cassette on a radiator, in the sun or in a stove.

This method is somewhat impractical because the cassettes have to be weighed at regular intervals.
  1. Weigh the cassette in order to determine how much its weight needs to be reduced.
  2. Put the cassette on a heater, in the sun or in a stove. Make sure that the temperature does not exceed 70° Celsius in order to best preserve the properties of the silica gel.
  3. Weigh the cassette at regular intervals and remove it from the heating source when the desired weight has been achieved.

B - Drying with the aid of desiccant bags.

The desiccant bags DESI PAK (desiccant clay) come in different sizes and are very inexpensive. Between 0% and 40% RH, one 35 g bag absorbs 6 g of water vapour.
  1. Weigh the cassette in order to determine how much its weight needs to be reduced.
  2. Put the cassette in a polyethylene bag, together with the required amount of desiccant bags.
  3. Weigh the cassette after some days to see that the desired weight has been achieved. Please Note: The drying agent can be regenerated by heating the bags in a stove at 110° - 130° Celsius. There is no risk of heating them for too long.

C - Drying with the aid of a desiccant bag inside the display case.

This is often the most practical way of keeping display cases within a certain relative humidity range over longer periods of time.
  1. Put the desiccant bags directly to the side or underneath the cassettes while they are in the display case. It is recommended not to add more than one or two 35 g bags at a time per cassette, in order to not reduce the RH inside the display case too abruptly.


You were using PROSORB inside your showcase but the RH inside the showcase is different from what you expected. There can be several reasons:


Print this page Print this page

Long Life for Art | Christoph Waller | Hauptstr. 47 | D-79356 Eichstetten | Tel. +49(0) 7663 608 99-0 | Fax -20
E-Mail:, Web:, © c.waller