In January, the conservation team prepared some purified isinglass. Isinglass is a type of glue made of fish, more specifically the dried sturgeon swim-bladder membrane. This adhesive offers different qualities for conservation treatments, such as good ageing properties, flexibility and light fastness. Isinglass is widely used for conservation treatments, for example the consolidation of pigments, repair of parchment or prepared as remoistenable repair tissue.

Figure 1_Weighting isinglass
Figure 1: Weighing isinglass

However, isinglass can’t be used in its raw form. Starting with sheets of the fish membrane, the isinglass must be dissolved, purified and prepared in a form ready to use. The full procedure takes approximately 3 days.

After weighing the desired amount of dried glue, the sheet is carefully cut in small pieces of a few millimetres length. The pieces are covered and left to soak overnight in deionised water.

Figure 2_The membrane is cut out in small pieces
Figure 2: The membrane is cut out in small pieces

The glue is then sieved, gently massaged then, divided into equal parts, and put to dissolve in fresh deionised water. The water is gently warmed in a bain-marie at 29 degrees, and frequently stirred up to facilitate the dissolution. At higher temperatures, the gelatine of the glue starts to degrade and its structure and properties are then altered.

When the dissolution is completed, the isinglass is sieved twice through a thin muslin cloth to remove any impurities. The discs can then be prepared!

Using pipettes, small drops of glue are carefully spaced out on a sheet of Melinex©.

Figure 7_Isinglass drops on Melinex©
Figure 7: Isinglass drops on Melinex©

The drops need to dry whilst covered, protected from dust and impurities. This takes usually between 12 to 24 hours depending on the weather conditions.

Figure 8_Drying under a plastic cover
Figure 8: Drying under a plastic cover

The discs can finally be peeled out of the Melinex and stored in a jar.

The discs are now ready to be diluted in water, warmed up in a bain-marie and used as an adhesive in a conservation treatment. Keep an eye out for our next blog to see it in action!