In practice the working of an evaporator is often not a very difficult matter, and large numbers of machines are operated by unskilled labour.

Troubles generally arise from inconstant steam pressure, incrustation, leakages of air, which reduce the vacuum, the temperature head, and hinder heat transmission. For the evaporation of gelatine liquors the Yaryan, the Kestner, and the Blair-Campbell film evaporators are the most widely used. The velocity of the liquor through some of these machines is so great that occasionally no vacuum is used. The temperature obtained is high (200 F.), but the time is very short, if rapid cooling of the evaporated liquor is arranged.

REFERENCES.

"Evaporating, Condensing and Cooling Apparatus," by E. Hausbrand.

Scott, Greenwood & Son (1916 Ed.).

"Evaporation," by E. Kappeschaar. Norman Rodger (1914).

"Evaporation in the Chemical Industry," by J.A. Reavell, M.I.Mech.E., _J.S.C.I._, 1918, April 11th.

"Glue and Glue Testing," S. Rideal, D.Sc., pp. 56-59.

"Gelatine, Glue, and their Allied Products," T. Lambert, pp. 26-29.

"Notes on Condensing Plant," J.M. Newton, B.Sc., _J. Junior Inst._ Engineers, Aug., 1912.

SECTION VII.--COOLING AND DRYING

The conversion of a gelatine sol into cakes of gelatine has been much simplified by the advent of the evaporator. Before this machine was used much trouble was experienced with putrefaction, and in hot and thundery weather, especially on the Continent, it was often necessary to suspend operations. Evaporation has, however, materially contributed to the possibility of rapid and satisfactory cooling and drying.

From the time the weak sol is decolorized and bleached, the finishing processes consist essentially in the removal of water. This is now usually done partly by evaporation of the sol, and partly by the desiccation of the gel. There is an obvious elasticity in method, and factory practice does actually vary considerably in the relative proportions of these two alternatives. Some factories evaporate to a 20 per cent. sol, approximately, and rely upon drying sheds and lofts to complete the desiccation: other factories evaporate up to a 55 per cent.

gelatine sol, and so can manage with less shed room. Something depends upon local conditions, but the main issue is between the cost of steam in evaporation and the cost of land and buildings required for sheds. On the whole the modern tendency is to evaporate more, for this course has the additional advantage of speed, involving both a quicker turnover and less liability of putrefaction. Lower-grade products need relatively greater evaporation to form a gel of equal rigidity.

After evaporation and bleaching, the concentrated sol is first cooled rapidly until it has set to a stiff gel, then cut up into cakes according to the size required, these being dried out on network frames arranged in tiers, through which a draught of air is usually forced or induced. This general description is of course applicable to many factories with innumerable variations in detail, most of which variations originate in local convenience and are unessential parts of the manufacture.

An essential principle is that the cooling or gelation should be done rapidly, not only to avoid putrefaction but also to avoid the action of heat on the elasticity of the gel. A hot sol or gel is liable to hydrolysis and loss of setting power, and should have its temperature quickly reduced, but a warm sol or gel (say 100 F.) is most liable to putrefaction, so that the cooling should be continued quickly. On the other hand, the gel should not be frozen. For cooling purposes a copious supply of cold water is most usually employed, but some factories have installed refrigerators. These plants operate by the rapid evaporation of liquefied gases such as carbon dioxide, sulphur dioxide, or ammonia, so arranged as to cool a solution of common salt, which forms the circulating liquor and is returned after use to the refrigerator. Where such plants are used, it is natural that their use should be extended to the drying sheds to cool the air entering in the height of summer. In some factories the cooling is attained neither by cold water nor cooled brine, but merely by cold air.

The kind of vessel in which gelation is induced varies widely in different factories. For lower-grade products metal boxes are used, heavily galvanized iron being the most common material. If the liquor be muddy, deep boxes are preferred, but if clear, rapid cooling is best attained by having them long and shallow, and so exposing a relatively greater area to the cooling action. In either case the boxes may contain up to 1/2 cwt. of jelly. Lambert mentions boxes 24" 6", which are 5"

deep; Cavalier suggests rectangular moulds holding 30 litres. In place of galvanized sheet iron, boxes of sheet zinc or of wood lined with zinc are sometimes used. In any case the most scrupulous cleanliness should be observed in all cooling-house work, and in some factories the most elaborate precautions are taken for cleansing vessels, tools, floors, etc., and even for their disinfection and sterilization. Iron, tinned iron, and copper cooling vessels are ruled out on account of their tendency to rust and tarnish, and the last is unjustifiably expensive.

Many of these vessels are unsuitable for pure food gelatines in which traces of copper, zinc and a.r.s.enic are held to be very objectionable.

For the best gelatines, therefore, a very shallow vessel (1/4" to 1/2"

deep) with a sheet gla.s.s bottom is preferred, and the concentrated sol is run on to this for gelation.

Glue (or gelatine) which has set in this way is sometimes called "cast glue." That which sets in metal boxes in blocks is termed "cut glue,"

because the blocks of jelly need subsequently to be cut into slabs of the desired size and shape. Jelly blocks may be cut by hand with the "wire knife" which yields a characteristic wavy appearance to the finished product. This may also be done by machinery, the block of gel being placed on a series of correctly s.p.a.ced wires and forced through the network by hydraulic pressure. A cutting machine (Schneible) has also been used to cut up blocks of jelly into slices of the required thickness, but these machines have not made great headway in this country. It will be clear that cast glue is cooled more rapidly than glue in blocks; it is therefore not surprising to note Lambert's statement that the former comprises the larger proportion on the market.

The cut or cast cakes are next placed upon network frames, and a series of such frames are placed on a bogey. The bogey is run along tram lines into the drying tunnel, through which air is forced or induced by a fan.

Many such bogeys are, of course, pa.s.sed into each tunnel, and as many tunnels as required may be constructed. Care is necessary to expose the cakes evenly to the action of the air. It is mostly necessary to warm the air at the inlet by means of steam pipes and so increase its drying power. This is especially necessary in winter or wet weather. In summer, however, it is often arranged that the air is cooled before entering the sheds. This is accomplished by pa.s.sing the air through pipes from a refrigerator. When heated air is used, it is stated by Lambert that the maximum temperature should be 25.5 C. (78 F.); Rideal considers 21 C.

(70 F.) should be the maximum. In all cases the drying power of the air is easily ascertained from a wet-and-dry bulb thermometer, and the amount of air pa.s.sing along the shed from a wind gauge. Lambert states that drying normally occupies four to five days. The final product is still a gel, of course, and contains from 10 to 18 per cent. of water.

It appears, however, very hard and solid. The dried cakes are removed from the frames and transferred to the warehouse, where they are sorted according to quality and packed in bags or tin-lined boxes. Some material is ground to powder.

The network of the drying frames has been made from many materials.

Cotton or string netting is very common, but is liable to sag and to get dirty. It also has a short life. Ordinary galvanized iron soon loses its galvanizing cover, and the iron then is liable to rust. Attempts have been made to use sheet zinc and other alloys, which are cut or punched into nets with square or diamond-shaped holes. These were found to warp and break. Rideal's conclusion, which is confirmed by the general experience, is that the best material is a heavily galvanized iron wire netting. He suggests that it should have 15 to 25 per cent. of its weight of zinc, and that it should be strengthened by stiffer ribs arranged both longitudinally and transversely.

Many attempts have been made, and many patents taken out, with the object of making the cooling, cutting, and drying processes as continuous and as quick as possible, and with a view to saving labour, which is rather costly at this stage. These attempts, however, have only met with indifferent success. A common idea is that a continuous supply should fall upon a revolving appliance, and be instantly congealed in a thin state, which last lends itself to more rapid desiccation. Vacuum drying has also been attempted.

REFERENCES.

"Glue and Glue Testing," S. Rideal, D.Sc., pp. 68-74.

"Glue, Gelatine, and Allied Products," T. Lambert, pp. 30-35.

_Chem. Zeit._, 1911, 85, 17 (Cavalier).

PATENTS.

Eng. Patent (1894) 11,426 (Hewitt).

Eng. Patent (1898) 2,400 (Brauer).

Fr. Patent (1909) 398,598 (Lehmann), _J.S.C.I._, 1909, 897.

U.S. Patent (1912) 1,047,165 (American Glue Co.).

SECTION VIII.--USES OF GELATINE AND GLUE

Gelatine and glue have both been put to an immense variety of uses, and the list is constantly extending. Indeed, no one who considers the following account of their applications can doubt that gelatine and glue have become a necessary part of our civilization.

Gelatine for edible purposes certainly forms a very considerable part of the total used, and great pains are now taken to obtain a pure product.

Thus, a gelatine with more than 1.4 parts per million of a.r.s.enic, or more than 30 parts per million of copper, is not considered good enough for "pure food." The food value of gelatine, compared with other proteids, is exceedingly low; its use in this connection has no connection with the "calories" of heat energy it will yield. It is used almost entirely because of its property of forming a gel. Table jellies form, of course, one popular use of gelatin, but the manufacture of sweets makes also a great demand upon the gelatine trade. Culinary operations often require a little gelatine, especially is it used in pies and soups. An extension of the same idea is found in its employment for many manufactured foods, _e.g._ tinned meats, meat extracts, and the concentrated foods. The use of gelatine in connection with the first of these received a big impetus during the war period. In gelatine for any of these purposes, the presence of excess of sulphurous acid is objectionable, as its taste is easily noticed.

Gelatine for medicinal purposes finds an ever-growing number of applications. Gelatine capsules for holding greasy liquids and solutions of nauseous drugs are increasingly popular, for the dose may be swallowed without unpleasantness. In making these capsules some sugar is also used, and the finished article is often protected from atmospheric moisture by treatment with a weak solution of alum. In a similar way pills are often coated with a 33 per cent. gelatine sol.

Such pills are not only pleasanter to swallow, but are less liable, after being dried, to stick together in the box. Alcohol solutions of drugs (or essences, perfumes, etc.) may be suitably stored in gelatine instead of metal tubes. Medicated wines are detannated by gelatine before the addition of drugs which would have been precipitated by the tannin. The British Pharmacopoeia specifies four kinds of "Lamellae,"

which are small discs of gelatin and glycerin, each containing a minute but definite dose of some powerful alkaloid. Glycerin jelly is a mixture of gelatin glycerin with some water. It is used for chapped and rough hands; the mixture is also used for glycerin suppositories, and for mounting microscopic sections. The mixture also forms the basis of gelato-glycerin, used in nasal bougies, and of glyco-gelatin for medicated lozenges. Gelatine insolubilised by formalin (formo-gelatin) has been used for making tabloids, wound dressings, and artificial silk.

Gelatine is in constant demand for bacteriological work, for which purpose a high-grade product is desired. Nutrient media for the culture of bacteria are solidified by 10-15 per cent. of gelatin, and the growth of colonies of bacteria often show typical formations. By inoculating into a melted and sterile quant.i.ty and setting quickly in a flat dish after mixing, the number of bacteria in the volume introduced can be judged from the number of colonies which develop. Bacteria are also distinguished often as "liquefying" or "non-liquefying" according to their type of culture on nutrient gelatine media. Gelatine for such work should be neutral and of high clarity.

The gelatine required for photographic purposes is also a high-cla.s.s product. It should be neutral, colourless, and free from chlorides and other mineral salts. Grease also is objectionable. Gelatine is used in the numerous carbon processes, in which the principle is that gelatine is made insoluble in water by the action of pota.s.sium dichromate under the action of light. It is used also in Poiteoin process for copying engineering drawings, which is based upon the power of a ferric salt to render gelatine insoluble so long as it is not exposed to the actinic rays.

Gelatine is used in the manufacture of the "crystalline gla.s.s" used for decorative purposes. Advantage is taken of the immense contractile force it exerts on drying. When ground gla.s.s is coated with gelatine, and the latter dried, it tears away the surface of the gla.s.s itself, and leaves peculiar fern-like patterns. Inorganic salts dissolved in the sol influence the nature of the pattern obtained.

Gelatine is used also very largely in the textile trades, for finishing coloured yarns and threads, for sizing woollen and worsted warps, and for thickening the dyestuffs used in printing fabrics. It is also used for finishing white straw hats; as a size in the manufacture of high-cla.s.s papers, and as a wax subst.i.tute for covering corks and bottle necks.

Glue is used instead of gelatine in all cases where colour is not a matter of much moment. The fact that it has not been bleached makes no difference to its suitability in such a case, and the cost is substantially reduced. Thus, for dark-coloured straw hats, textiles, sweets, papers, and in all suitable woolwork, glue is used in place of the more expensive article.

A very large quant.i.ty of glue is used in the manufacture of matches, where it functions as the material binding the "head" to the stem. A 15-50 per cent. sol is used, containing nitrate or chlorate of potash as oxidizing agent. The mixture is kept at 38 C. and the phosphorus cautiously added, and when this is emulsified, the friction ingredients (sand, gla.s.s, etc.) are also added. The glue acts also in preventing premature oxidation. Glue is also used in making the match-boxes, and similarly in making sand, emery, and gla.s.s papers and cloths.