CHAPTER X.

ISINGLASS AND ITS SUBSTITUTES.

Isinglass is obtained from the air-bladder or sound, as it is sometimes termed, of different kinds of fishes, especially of the sturgeon, species _Acipenser_. It is used for culinary purposes, fining beer and other liquids, for making court-plaster and stiffening silk, though a good quality of gelatine is practically equal to it for these purposes. A good quality of isinglass should be pure white, semi-transparent, dry and horny in texture, and free from odor. It should dissolve in water of 95° to 122° F., without leaving any residue, and, on cooling, should yield an almost colorless jelly. From gelatine imitations it is distinguished by soaking it in warm water and examining under the microscope, when true isinglass will show a net-work of long, curling fibres, while gelatine will be simply hyaline. Isinglass is often imitated with the intestinal membranes of the calf and of the sheep. This spurious article may be readily recognized because it does not exhibit, like isinglass, a sort of shining appearance when held before the eye and daylight, and because, although inodorous, it has a saltish flavor. If it be torn asunder it will be observed that it may be rent in all directions, while true isinglass cannot be divided otherwise than in the direction of its fibres. If a piece of artificial isinglass be macerated in water it swells, but instead of retaining its shape as is the case with the genuine article, it becomes divided into several pieces, forming a sort of curdy precipitate; and if treated with boiling water, about one-third of its weight is left in an insoluble state, and the liquor does not form a good jelly. Isinglass is frequently adulterated with gelatine, which is inserted between the leaves and rolled up with it. The best indication of this adulteration is the amount of ash; isinglass yields only 0.9 per cent., while gelatine yields 4 per cent., and adulterated isinglass 1.5 per cent. or more.

1. _Russian isinglass._ Russia produces the best and most isinglass. It is chiefly obtained from several varieties of sturgeon, species _Acipenser_, which inhabit the Caspian and Black Seas, and their tributary rivers. The _Acipenser Gueldenstaedtii_, Br. yields the finest, best and whitest isinglass. It is known by the name of _Patriarch_, and consists of small horseshoe-shaped pieces rolled tightly together. It is quite scarce and expensive. When the bladder is merely dried in sheets, it constitutes _leaf isinglass_. When several bladders are put together and folded before they are completely dry, they form _book isinglass_. Each bladder may also be rolled up and folded around a few pegs in the form of a horseshoe, heart, or lyre, in which shape it is dried. The latter is the _staple isinglass_, which, according to its dimensions, is again divided into _long and short staple_.

Long staple isinglass of fine quality is produced in the Oural. It is imported in loose leaves, and at times it is twisted like ropes, this kind being preferred, as it is inferior in quality only to Patriarch.

_Siberian_ purse isinglass is of moderately good quality and is in general demand. A small kind of strings in a necklace form is sometimes imported.

A very good sort of Russian isinglass comes into commerce in leaves and books, and is known by the name of _Samovey_ leaf. It is obtained, according to the statements of Russian merchants, from the common sheath-fish (_Siluris Glanis_). The pieces are as large as a hand, of the thickness of pasteboard, very solid, not very flexible and of a white-yellowish color. It is inferior in quality to Astrakhan isinglass, which is one of the best kinds.

In _Russia_ the isinglass is generally prepared by boys under the supervision of elder experts. The swimming bladder is first placed in water and left there for some days with frequent changes of the water and removal of all fatty and bloody particles. The warmer the water the more rapidly the operation is completed. The bladders are finally removed and cut longitudinally into sheets which are exposed to the sun and air, being laid out to dry, with the outer face turned down, upon boards of linden or bass wood. The inner face is pure isinglass, which, when well dried, can with care be removed from the external lamellæ. The finer sheets thus obtained are placed between cloths to protect them from the flies, and are then subjected to a heavy pressure so as to flatten them out and render them uniform. After this they are assorted and tied up in packages. The packages composed of the isinglass of the large sturgeon usually contain from ten to fifteen sheets and weigh a pound and a quarter; those of the other contain twenty-five sheets weighing a pound. Eighty of these packages are usually sewed up in a cloth bag, or sometimes inclosed in sheet lead.

The outer lamellæ of the air-bladder, after the isinglass has been removed, also contain a considerable quantity of glue which, when softened in water, is scraped off with a knife and moulded into little tablets of about the size of a silver dollar, and then dried.

2. _North American or New York Isinglass._ It is in thin strips several feet long but ½ to 1½ inch wide. It is less soluble than Russian isinglass, and yields frequently a dark-colored solution. It is prepared, according to Dr. J. V. C. Smith’s statements, from the air-bladder of the common hake (_Gadus merluccius_), which is macerated in water for a short time, cut open and subjected to pressure between iron rollers, by which it is elongated to the extent of half a yard or more. It is then carefully dried, packed and sent to market. The air-bladder of the common cod (_Gadus morrhua_) is prepared in a similar manner, but yields a poorer kind of isinglass.

3. _East India Isinglass._ It would seem that for a long time this has been exported from _Calcutta_ to _China_, but has only lately attracted the attention of European dealers. It is prepared from the air-bladder of the _Polynemus plebejus_, and comes into commerce either in the form of leaves or purses which seem to consist of the unopened air-bladder. East Indian isinglass has a disagreeable fish odor, due very likely to careless preparation, which makes its use impossible for many purposes, and, of course, depreciates its commercial value. The oval-oblong purses are about nine inches long, three and a half inches wide, weigh about 7 ozs. and have a dark-yellow color. East India leaf isinglass, _i. e._, the opened and dried air-bladder, consists of yellowish-colored leaves eight to nine inches long, six to seven inches wide, and about three-tenths of an inch thick. The leaves are sometimes rolled out into long ribbons about one-tenth of an inch thick, the surface of which is covered in places with a thin film of lime.

What is known as picked East India isinglass is brought into commerce in small shreds about two to three inches long, and tapering at the extremities.

A variety of isinglass very white and pure and scarcely inferior to Samovey leaf is brought from Manila. The fish which yields it is caught on the coast of the Philippine Islands, especially at Luzon.

4. _Hudson Bay Isinglass._ It is brought into commerce in the purse form. Some specimens measure twelve inches in length and three and a half inches in diameter, and weigh one and a half ounces. It is of light-yellow color, nearly transparent, without odor or taste. The inner lining of the sac, which can be readily stripped off, is insoluble in water, while the remaining portion dissolves to a slightly colored jelly. We have been unable to ascertain from what species of fish this isinglass is procured.

5. _Brazilian Isinglass._ This is imported from _Para_ and _Maranham_, and is also called _Cayenne_ isinglass. For a long time there existed a doubt from what species of fish this isinglass was procured, but it is now settled that it is prepared from the air-bladder of _Silurus Parkerii_, a fish which is frequently found in the muddy waters of the rivers in the province of Grao Para, where these waters mingle with the sea.

_Brazilian_ isinglass comes in the form of pipe, lump and honeycomb. On account of its dark color it is not in much demand for ordinary use, but is frequently employed in England for clarifying glue. When digested in water it leaves much insoluble substance behind, being in this respect also inferior to Russian isinglass.

6. _German Isinglass._ Under this name we may mention the mucous membrane of the sturgeon (_Acipenser sturio_), prepared in Hamburg. When boiled with water it leaves 16 per cent. of insoluble substance.

It is said that an excellent isinglass can be made from the scales of shad and herring, which are first freed from their silvery coating. This may furnish a useful hint to persons who are near some of the great fishery establishments of the country, at _Gloucester, N. J._, and _Alexandria, Va._, for instance, where thousands of shad are scaled and salted every year.

To give inferior qualities of isinglass a better appearance and make them more salable, they are frequently bleached with sulphurous acid.

_Ichthyocolle Française._ Under this name, Rohan has introduced a substitute for isinglass. The material used for its manufacture is blood fibrin, which, after washing in running water, is thoroughly kneaded and, after draining, digested at 59° F. with dilute sulphuric acid of 8° to 10° Bé. for eight days, after which the mass is freed from acid by washing in running water.

The fibrin freed from acid becomes transparent and gelatinous by treating with weak soda lye of 3° to 4° Bé. at 59° F., whereby it swells up and increases hourly in volume. After twenty-four hours it is taken from the soda lye, and after removing the free soda by washing, heated to 212° F, in a water-bath. The fibrin dissolves and becomes so thinly fluid that it can be filtered. 75 to 80 per cent. of the water is then evaporated, and the fibrin thus prepared can be used as a substitute for isinglass for fining purposes. Ichthyocolla swells more quickly in cold water than isinglass; 15 to 20 per cent. divided in water forms a thickly fluid substance, which on heating dissolves to a perfectly clear fluid. For fining beer with ichthyocolla add 2 to 10 per cent. of pure tannin, which does not injure its power of dissolving.

_Isinglassine._ Under this name is known an isinglass substitute prepared from the gelatinous material from calves’ feet and other sources. The material is reduced by machinery to a pliable homogeneous mass, rolled out into sheets, dried, pressed and shredded.

_Chinese isinglass_ is identical with the _Japanese Agar-Agar_, and is obtained by cleansing and boiling certain species of algae found in Chinese and Japanese waters. This isinglass, or gelatine, possesses the following properties:

Placed in cold water it softens without dissolving like gelatine, and forms a structureless tubular mass which is not sticky. By boiling, it dissolves more readily than isinglass, but with greater difficulty than gelatine. A 1 to 2 per cent. solution is easily filtered through paper or linen, and when cold forms a solid jelly clear as water and without taste or odor. Jelly prepared with one-half per cent. of Chinese gelatine is more solid than one prepared with 4 per cent. of French white gelatine, retains its consistency longer, and will stand a temperature of 86° to 122° F. before becoming liquid. Used for jellies, or mixed with other foods, it does not impart to them a glue taste never wanting in bone gelatine. When decomposed by long standing, it does not acquire a disagreeable odor, while decomposed isinglass or gelatine exhales a putrid smell.

Agar-Agar contains, according to analyses: Cellulose, starch, gum, dextrine, vegetable mucus, vegetable wax, resin, chlorophyll, albumen, a peculiar acid, and several mineral substances.

_Irish moss_ (_Chondrus crispus_) which grows on rocks of the American and European shores of the Atlantic Ocean, yields a jelly which has been employed as a substitute for isinglass, as a size, for thickening colors in calico printing, and in stiffening silk. In a fresh or softened state the plant is cartilaginous, of a brownish or purple, or frequently yellow or green color. After washing in water and drying in the sun it turns whitish or yellowish, and becomes somewhat translucent and of a horny appearance. It has a slight seaweed-like odor and a mucilaginous, somewhat saline taste. One part of it boiled with 20 parts of water gelatinizes on cooling.

_Fish Glue._

This product, which is prepared in many localities from fishes, must not be confounded with isinglass, though the purer varieties may serve as substitutes for the latter or for gelatine.

Jennings gives the following process for the preparation of fish glue. The fishes are treated with dilute sulphuric acid until the skin can be detached. The acid water is then drained off and replaced by milk of lime to neutralize adhering sulphuric acid as well as to saponify the fat. The milk of lime is several times drained off and renewed, the mass thoroughly washed, cut up in a hollander and treated cold with solution of sodium hyposulphite, common salt and alum. After a few days the liquor is drawn off and replaced by a mixture of alum solution, dilute sulphuric acid and nitric acid in which the mass is allowed to remain for a few days. Fishes with dark skins are treated with a mixture of hydrochloric and sulphuric acids. After washing, the skin is removed and the fibres which have become detached from the bones are separated by digestion in dilute solution of mercuric chloride and alum. Adhering fatty parts are removed with warm milk of lime, the lime is neutralized with hydrochloric acid, and the mass boiled with water for the formation of glue. The resulting glue-liquor is clarified with sulphurous acid and alum, and when all the impurities have subsided, compounded with acid sodium carbonate till all the acid is neutralized. The finished solution is concentrated so that it gelatinizes on cooling and can be cut in cakes which are dried in the usual manner.

Fish scales, especially those of carps, are treated in a similar manner. The bone-earth is extracted with hydrochloric acid, the extracted material thoroughly washed, and then boiled in soft water till it can be readily stirred. The liquor is drawn off from the horny sediment, clarified with alum, evaporated, and after all the impurities have subsided, poured into moulds and treated like ordinary glue.

Considerable quantities of fish glue are produced on the Norwegian coast from waste obtained in the preparation of codfish. The fish when caught are cut open and the air-bladders removed, which are dried and brought into commerce as isinglass. The head is then cut off and the bones detached in one piece. The flesh is dried in the air, and forms the codfish of commerce. The heads and bones are first treated with hydrochloric acid or directly boiled under slight pressure in water, and the resulting liquor concentrated so that it will gelatinize.

A substitute for isinglass, also for gelatine and glue, is prepared by C. A. Sahlströhm, of Stockholm, according to his patent, from fish and fish waste by treatment with bleaching powder, potassium permanganate, and nitrous and sulphurous gases.

For this purpose the fishes, or portions of fishes, are first well washed in fresh water and then left for from three to four hours in a solution of bleaching powder (in the proportion of 2 lbs. of bleaching powder to 300 quarts of water). After washing they are treated for about 30 minutes with a solution of potassium permanganate (in the proportion of 1¾ ozs. of potassium permanganate to 250 or 300 quarts of water), and then exposed to the action of the nitrous gas, produced by heating 10 to 15 ozs. of nitric acid for every 88 lbs. of raw material. This gas may be first absorbed by water, as in the manufacture of sugar, or sulphur dioxide may be used instead of nitrous gas. The former would be obtained by burning about 7 ozs. of sulphur for every 88 lbs. of raw material.

The material, after this treatment, is washed. Those portions intended for the production of isinglass substitute are freed from their outer skins and dried and pressed at a gentle heat. The portions destined to produce gelatine or glue are, on the contrary, exposed to a temperature of from 104° to 122° F. for from ten to twelve hours, by which the material is mainly dissolved. The mass is then forced through a strainer or sieve, and the liquor allowed to gelatinize by standing for some hours. The jelly is finally dried, as is usual in the manufacture of glue or gelatine.

_Whale glue_ is, according to Culmann, obtained in the Russian island Jeretike from the liquor remaining in the boilers after the extraction of the oil by means of superheated steam. By reason of the raw and moist atmosphere which prevails in that locality, even in summer, drying the glue is not practicable, and for this reason it is brought into commerce in the form of a compact jelly mixed with a preservative and packed in tin boxes. The commercial article contains 41.65 per cent. water. It is liquefied by placing the can in boiling water, and melts at 176° F. As shown by mechanical tests, it possesses great tenacity, and two pieces of wood glued together lengthwise cannot be separated at the joint but break alongside of it.