Glue can be extracted from fish by heating the skin or bones in water. The purest form of fish glue, made from the membrane of the air bladder (swim bladder) of certain species of fish such as the sturgeon, is also called isinglass. Isinglass can be produced from various species of fish using diverse manufacturing processes.
Tatyana Petukhova, a paper conservator at Cornell University Library in Ithaca, New York. She wrote an article, The History of Fish Glue as an Artist’s Material: Applications in Paper and Parchment Artifacts. It was published in “The Book and Paper Group” (Annual Volume 19, 2000) and can be found in its entirety at http://cool.conservation-us.org/coolaic/sg/bpg/annual/v19/bp19-29.html
Here are two sections from her article that discuss the illumination of parchment manuscripts and experimentation with fish glue by 19th century artists.
“Illuminated Parchment Manuscripts
“In medieval Europe, parchment was the main material for writing. It was usually made from sheep or calf skin, but occasionally from the skins of such other animals as goat, antelope, and gazelle. Preparation of the parchment was a time-consuming procedure requiring special skills. One of the many steps in the process was sizing of the parchment, which enhanced its strength and prevented the writing medium from penetrating too deeply, allowing the parchment to be reused. When the parchment was to be used over again, the old ink or gouache-like medium was removed from the surface by rubbing pumice over it; the area was then softened so it could absorb new writing.
Two types of size application, coating and impregnation, were employed. The sizing solution was generally produced from scraps of parchment or trimmings of the whole skin of an animal. Small pieces were then soaked and boiled in fresh water. Fish glue was also used to size parchment.
Gouache on Paper and Board
In the nineteenth century some artists experimented with non-traditional techniques. The gouache paintings by M. Pierran, for example, which were coated heavily with glue in order to obtain a special effect, were exhibited in the 1834 Salon. These gouaches with their glossy surfaces resembled oil paintings. The technique involves application of the mixture of gouache with a large amount of gum and fish glue. These paintings over time have developed delamination and cracking.
Another method of painting with watercolor on specially prepared Bristol board was developed by C. J. Robertson, for which he received the Medal of Isis from the Society for Encouragement of the Arts, London. The process, from the backing of a Bristol board to the coating, was elaborate. When the picture was completed it was “varnished” with a solution of fish glue and then with a good quality picture varnish. “The advantages of the method are that the color, which stays very brilliant and transparent, may be worked over in a way impossible by any ordinary method. A similar method is described by Vibert.
Artists’ experiments with coating and glazing of paintings and drawings with fish glue were recorded as early as the seventeenth century. Fish glue produced by boiling of the swim bladders of sturgeons was experimentally used by Van Dyck in his tempera paintings. When fish glue was applied in many layers and in glazed coats, the film formed was easily chipped off.
These earlier attempts demonstrate that fish glue used alone forms a brittle film. As with any other adhesive, when and where it is appropriate to apply should be considered carefully. Perhaps the brittleness of the film formed by this glue motivated artists to introduce various plasticizers that are also used in conjunction with fish glue in restoration. Molasses in England and honey in Russia have often been used as natural plasticizers. For example, isinglass glue mixed with honey had been used for the consolidation of delaminated paint in Russian icons as early as the seventeenth century.”
