If they were so good, why do we not see a lot more on eBay? They were, after all, produced from the 1930s until, probably, the 1980s.

The main reason is their cost. They were top of the market products. Let’s take a price comparison from the 1950s. Rondinax, Essex and Kent tanks were four times the price of a simple conventional vertical tank with spiral.

In the 1952 BJPA, the prices of Johnsons of Hendon developing tanks were quoted. A simple universal tank was £1-9-6d; the Essex Mark 1 was £4-18-6d and a Mark II, £5-18-6d. In 2013 terms, those prices (calculated from R.P.I.) are £24.60, £100 and £127.80, respectively (Rondinax tanks when they became available were priced similarly to the Essex). Showing that this is a meaningful calculation, a new Paterson tank very similar to the Johnsons Universal, I see advertised at £21.98; the price in real terms has not changed in 60 years.

Photography in the 1950s and 60s was an expensive activity and any money that could be saved by the average photographer was saved. Another example is that war surplus photographic materials (bulk film, paper etc) were sold in bulk for many years from the back pages of Amateur Photographer.

Daylight-developing tanks were simply out of the reach of most photographers, especially when they had to have darkroom facilities of some sort in order to do the printing at home.

The second reason is that reversal colour processing became very popular, Ferraniacolor, for example. That process involved exposure of the film to light after the first development stage. The tank manufacturers were soon off the mark in making film spirals with translucent ends in order to let the light reach the whole film. In practice, the spiral containing the film was lifted out, immersed in a bowl of water (to prevent drying) and rotated gently by hand under a photoflood lamp held a short distance from the surface of the water. How many people were electrocuted while processing Ferraniacolor at home is not recorded.

Agfa were slow off the mark in making translucent ends to the spirals, although they did so eventually. The manual for the Essex said that the film could be removed from the spiral, exposed to light and then wound back on when wet. I am by no means convinced that many people tried that, trusting instead to darkroom-loading tanks like the Polly-Max, a universal tank taking films from 116 downwards, on sale for £1-12-6d.

So I think there was a double whammy: price and utility. Now, there is a third factor that has reduced the number of Rondinax, Essex and Kent daylight-developing tanks, as well as many of the early vertical ones as well. Bakelite breaks when dropped. I have bought several and seen others that have been carefully glued back together in order to protect what was, in its time, a significant item of expenditure.

The prices at auction in 2013 represent supply and demand. On the supply side, the overall number of tanks must be decreasing as breakages occur. How many there are languishing in attics and in boxes full of inherited household goods is anybody’s guess but the numbers from that source are bound to decrease with time. Demand will depend on how many photographers will continue to use film.

Rondinax 60s and Kent 20s are fetching higher prices than the tanks for 35 mm film. Is this because there are more medium-format film photographers at the moment?