Isinglass was originally made exclusively from sturgeon, until the 1795 invention by William Murdoch of a cheap substitute using cod. This was extensively used in Britain in place of Russian isinglass. The bladders, once removed from the fish and processed, are formed into various shapes for use.
Isinglass finings are used extensively as a processing aid in the British brewing industry to accelerate the fining, or clarification, of beer. They are used particularly in the production of cask-conditioned beers, known as real ale, although there are a few cask ales available which are not fined using isinglass. The finings flocculate the live yeast in the beer into a jelly-like mass, which settles to the bottom of the cask. Left to itself, beer will clear naturally; however, the use of isinglass finings accelerates the process. Isinglass is sometimes used with an auxiliary fining, which further accelerates the process of sedimentation.
Non-cask beers which are destined for kegs, cans or bottles are often pasteurized and filtered. The yeast in these beers tends to settle to the base of the storage tank naturally, so the sediment from these beers can often be filtered without using isinglass. However, some breweries still use isinglass finings for non-cask beers, especially when attempting to repair bad batches.
Although very little isinglass remains in the beer when it is drunk, many vegetarians consider beers (such as Guinness and almost all real ales), which are processed with these finings, to be unsuitable for vegetarian diets (although acceptable for pescetarians). A beer-fining agent that is suitable for vegetarians is Irish moss, a type of red alga also known as carrageenan. However carrageenan-based products (used in both the boiling process and post-fermentation) primarily reduce hazes caused by proteins, but isinglass is used at the end of the brewing process, after fermentation, to remove yeast. Since the two fining agents act differently (on different haze-forming particles) they are not interchangeable and some beers make use of both.
Isinglass finings are also used in the production of kosher wines, although for reasons of kashrut they are not derived from the sturgeon, as this fish is not kosher. Whether the use of a non-kosher isinglass renders a beverage non-kosher is a matter of debate in Jewish law. Rabbi Yehezkel Landau, in Noda BYehuda, first edition, Jore Deah 26, for example, permits such beverages. This is the position followed by many kashrut-observant Jews today.
When repairing paint that is flaking from parchment isinglass can be applied directly to that area which has been pre-wet with a small amount of ethanol. It is typically applied as a very tiny drop that is then guided, with the help of a binocular microscope, under the edges of flaking paint.
It can also be used to coat tissue or goldbeater's skin. Here isinglass is similar to parchment size and other forms of gelatin but it is unique in that as a dried film the adhesive can be reactivated with moisture. For this use the isinglass is cooked with a few drops of glycerin or honey. This adhesive is advantageous in situations where minimal use of water is desired for the parchment as the isinglass can be reactivated with an ethanol-water mixture. It also has a greater adhesive strength than many other adhesives used for parchment repair. (Quandt, 1996)