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According to Wikipedia isinglass is a substance obtained from the dried swim bladders of fish. It is a form of collagen used [nowadays] mainly for the clarification of wine and beer. It can also be cooked into a paste for specialized gluing purposes.
Nowadays there seems to be the common belief that only waterglass was ever used to preserve eggs, and that anyone who thinks otherwise is mistaken. However, that is not so. Isinglass was regularly used to preserve eggs before the 1940s but due to the shortage of fish and fish products during the war years, waterglass became the preferred preserving agent.
The following method of how to preserve eggs with isinglass comes from someone who actual saw it done on a regular basis during the rationing and shortages of the Second World War. During this time, it was common to keep chickens, as it had been for generations in rural areas, so there was frequently a surplus of eggs which had to be preserved.
The eggs were dipped into the isinglass with a special egg clamp which was rather like a pair of spectacles without lenses in thin wire frames, bent toward each other and hinged on the nose bridge. Some of today's kitchen tongs are similar.
After dipping, the eggs were placed on a rack to dry. The rack was simply a flat piece of wood on two legs, with a dozen or so holes in it, each slightly smaller in diameter than an egg. Once the eggs were dry, the rack and its contents were kept in the garage on top of the food safe.
I don't know how long the eggs would keep, but it was always normal practice to break an egg open over a cup before using it. The smell of the egg would indicate whether or not it was still edible.
Incidentally, one night thieves broke into the garage and made off with Grandpa's tool case - together with the rack of eggs which must have seemed worth stealing at that time of rationing and shortages. Next morning, the police found the tools and case in a front garden further down the road - as it was probably too heavy to carry far - but there was a trail of broken eggs going down the road!
There are pictures of egg clamps on the internet, but they are very different from what Richard Ouston describes. They look more like regular clamps, but with points to do the holding. I suspect that they are used for holding eggs for painting.