By Didi van Trijp and Robbert Striekwold
On a gloriously sunny day in May we rang the doorbell of ichthyologist Martien van Oijen’s home in Leiden for a rather peculiar project. Even though the original plan had been to carry it out in a laboratory setting, on account of the beautiful weather we all agreed to move the project outside. In his garden, Martien had set up a table on which had been placed an array of equipment for dissection together with some specimens of red seabream that he had bought at the fish market that morning. The reason for all this? The replication of an eighteenth-century recipe for preserving fish skins.
This method was developed by Johan Frederic Gronovius (1690–1762), a physician and botanist based in Leiden. He boasted an extensive cabinet with all sorts of naturalia ordered according to the Linnaean system. In his quest for collecting specimens he developed a method for drying and compressing fish skins that would allow one to glue them to the pages of a book – much like dried plants in a herbarium. He described this method in a letter to Peter Collinson, who read it at a meeting of the Royal Society and had it published in the Philosophical Transactions in 1742. Compared to other ways of preserving fishes Gronovius’ method was quite easy and affordable, as few materials were needed. The only requirements were a pair of scissors, wooden plates, a linen cloth, ‘minikin pins’, and cartridge paper. Thus, according to Gronovius, “in the space of 24 hours, the fish is prepared.”
We set out to replicate Gronovius’ method step by step, carefully documenting each act with photographs and taking extensive notes along the way. The first order of business was to cut the fish open with a pair of scissors, while making sure that the fins were not accidentally destroyed.
Then most of its right half and all of the intestines were removed, which resulted in a rather gruesome sight.
Then we washed the left half and patted it dry with a linen cloth. After spreading the fins with pins, we exposed the half fish to the sun so that it could dry further (in the absence of sun, Gronovius recommended exposing it to the hearth).
We noticed fairly soon that some of the steps were not entirely clear to us. For one, what to do about the impressive swarm of flies that instantly flocked to the carcass once it was laid out to dry?
Most importantly, although Gronovius said the skin could be separated from the flesh “with very little trouble” after the drying step, it took considerable effort to do so. This may have to do with the fact that some of the steps are described in a somewhat ambiguous manner. We interpreted the step telling us the “back-bones are then to be cut asunder” to mean that the backbone should be cut but not removed.
After drying, however, these bones were very hard to remove, so we now think the entire backbone should be discarded before the drying step. Subsequent attempts with new specimens should shed more light on these issues.
The replication of this method is proving to be very insightful by giving us first-hand experience with a pertinent aspect of our respective projects: the preservation of fish specimens so that they could be collected, circulated, stored and classified. Fishes were notoriously difficult to preserve, losing their shapes, colours, textures, and often spoiling despite the collector’s best efforts to prevent these processes. So far, Gronovius’ method has indeed proven to be very quick and remarkably doable, and it appears to preserve the fish in very good shape, although we are not quite done with it yet.
After removing the dried flesh, the skin was placed between paper and left under a press overnight.
The next morning an elegantly flattened half-fish came out that would have done Gronovius proud.
Unfortunately, it did not smell quite as good as it looked. Our next step will be to use the recipe for a particular varnish that has been written up by Gronovius in a letter to one of his correspondents after having received a number of rotting fish skins. If all goes well, this should remedy the stench of the specimen – now safely stored in the freezer awaiting further treatment – and keep it in its current unspoilt state for many decades.
1 J.F. Gronovius, ‘A Method of preparing Specimens of Fish, by drying their Skins, as practised by John Frid. Gronovius M.D. in Leyden’ in Philosophical Transactions 42 (1742) 57-58.
 Robbert did the dissecting, Didi the documentation.