From the Chemicals Cupboard (Page Four)

Many photographic chemicals are sensitive to moisture. The most obvious is potassium
thiocyanate which will turn itself into a solution before your very eyes. This can also mean that
when you weigh it out, you are also weighing an intermediate amount of water. Seasoned weighers
and mixers usually are able to avoid this problem by keeping such compounds as solutions.
This is not always convenient. For example, when you are making up a working solution you have
to allow for a large amount of added water. The sketch shows a simple way of keeping such
materials dry. Two glass screw cap jars glued together ‘cap-to-cap’ in a sort of dumbell
formation, one containing your precious chemical and the other containing silica gel - hence the
Dumpot.

After you have stuck the caps together, cut the centres out of the sealing discs to leave a
pair of large washers, one for each cap, taking care to get a good neat edge to the hole. Put the
washer into the smaller cap, and drill as many holes as you can through the glued surfaces without
touching the washer. The idea is to give as large an area of hole as possible whilst providing
support for the disc of filter paper which keeps the chemical (or the silica gel) out of the holes,
but lows water to pass from the chemical half of the pot to the silica gel side. Cut a disc of filter
paper (I use Whatman No. I simply because I’ve got a packet — it needs to be as ‘coarse’ as
possible but no free fibres) to be a good fit inside each cap. The washer must cover any
irregularities in the disc edge.

The details of the jars will probably be determined more by what you have to hand than anything
else. I use 4oz polycarbonate ones for the silica (originally from Rayco) and 250g or 500g
jars for the chemicals. Use the smallest size that you can get away with so that you are drying as
little air as possible. Remember that each time you open the jar you need to dry another jarful of
air. Glass is best but even polyethylene (which lets a little water vapour through) will work well.
Use some indicating grade silica gel so that you can change it when necessary. By unscrewing one
half or the other you can get to the chemical or change the gel, and it is a lot less messy than
keeping small containers of gel inside the chemical jar. Keep a couple of original caps to
temporarily seal the jars while you do this.

                                            John Pearle.