Bone gelatine differs materially from skin gelatine, while the product of one animal may not be the same as that from another. Some gelatines - the inferior ones - dissolve at a low temperature, and others, again, set with extreme difficulty. It should be observed, says Dr. Eder, that good gelatine, when dissolved, not only colours water very slightly, but gives an almost colourless jelly. Dr. Eder suggests two practical ways of ascertaining the quality of gelatine. The first is - although it may not hold good throughout - to see how much water the material is capable of absorbing, the more water taken up the better being the gelatine. To find out this, a piece of gelatine should be accurately weighed, and then permitted to soak for 24 hours in water at 59° F. (15° C). The examination of a series of samples in this way will soon teach something about them. Another test is to find out the weight necessary to crush a gelatine jelly. Thus, if you have half-a-dozen samples to examine, solutions are in the first place prepared, 5 grm. gelatine being dissolved in 45 cc. water.
Half-a-dozen beakers or glass vessels of precisely the same diameter are obtained, and the solutions placed therein to set, at a temperature of 59° F. (15° G). All gelatines should set at this temperaturej if they do not, says Dr. Eder, they may be put on one side as unfit for photographic use. When set, there is lowered upon each jelly a little apparatus, consisting of a piece of tin shaped like a watch-glass, to the centre of which a wire is attached. The convex face touches the gelatine, and when it is weighted sufficiently it breaks through. At the upper end of the wire is a funnel, and to weight the apparatus small shot is dropped into the funnel. The weight of the apparatus should of course be in all cases the same, and this equality is soon brought about by adding shot to make up weight. The jelly which proves to be the firmest may be regarded as the best gelatine. An arrangement for steadying the wire is necessary, and this may be effected by covering the mouth of the beaker with a metallic plate having an orifice through which the wire passes. The moulded metal plate touching the gelatine need not be more than 1/2 in. in diameter, while such is the toughness of the gelatine in some cases that 3 lb. of shot are occasionally necessary before the apparatus tears the film.
The tougher the gelatine the better it is. The better kinds of gelatine are all found to dissolve pretty well at the same temperature, but in the case of gelatine of inferior quality this dissolves in water at a very low temperature.
It is not at all necessary, remarks Dr. Ballard, that glue-works should be a nuisance to the neighbourhoods in which they are situated. As respects the materials brought into the works: the moist materials, if not to be used immediately, should be at once placed in weak or old lime-pits or tanks, and in the event of an unexpected receipt of limed fleshings or pieces beyond the manufacturer's requirements for some length of time, it would be better (if the weather permits) to dry them off for future use than to leave them in loose heaps in the yard, especially in an open yard, and not under cover. At Turney's, in Stourbridge, moist fleshings are carefully stacked for future use. Before stacking the pieces in the winter, they are washed through a milk of lime in a washing machine. They are then stacked (about 100 tons in a heap) as closely as possible, so as to exclude the air. The stacking requires care. If any hollow places are left, the pieces become bad very soon. They are best put in large heaps 6 or 8 ft. high, since their own weight presses them down, and in a few days the heap becomes quite solid.
If at any time the sides or top of the heap become tainted, a layer of about 6 in. has to be cut off and re-limed. The practice adopted by some manufacturers, of preserving their fleshings immersed in lime liquor in sunken pits, is more injurious. An excess of lime has to be guarded against, since it destroys both the glue and the grease. The pieces which have been in lime for a long time yield much less than those boiled while fresh. Properly stacked pieces may without injury be preserved throughout the winter, or even for 12 months. Bevirtgton, of Bermondsey, another very large manufacturer, agrees with Turney. He says that in the event of a glue manufacturer being from any cause overstocked with "wet" goods, and being unable to use them fast enough, the best method is, broadly, to dry them; but this course is often impracticable for several reasons, such as (a) because, if the weather be bad for glue, it is at the same time bad for drying fleshings; (6) because of the want of proper appliances and space; and (c) because it depreciates the value of the goods, as when once dried they cannot be used for the same purposes as wet goods, e.g. for size-making. The method of drying being put out of the question, the next best thing is to stack them; and if this be done properly, they are but very little injured by keeping for several months, and are no nuisance whatever.
The way to do this is to place on a well-drained spot a layer of the fleshings a few inches thick, the size of the proposed stack, and then to throw over it a liberal supply of milk of lime, then put on another layer of fleshings, and treat it in the same way with milk of lime; and so on until all the goods are stacked. All this would appear to be to the interest of the manufacturer, and would certainly conduce to the comfort of his neighbours.
No good reason has been assigned for the universal practice of permitting the vapours from the boiling pans to diffuse into the atmosphere outside the sheds. There can be no more reason why this should be than it should be permitted to occur in the works of soap-boilers, trotter-boilers, etc, where methods of preventing the escape of offensive vapours from the works are in use at some establishments. Two methods of dealing with them may be suggested. One is the partial enclosure of the sheds in which the pans are situated, with the use of a fan to draw, off the vapours from the interior of the building to a tall chimney-shaft; and the other is the fitting of the pan with a cover provided with such a hinged lid as shall permit of the workman stirring the contents and skimming off the fat; conjoined with a flue carrying the vapours into a fire so arranged as to produce a down-draught into the pan. Dr. Ballard observed during his visits to glue-works that the vapour from the pans was least offensive when they were heated by steam, either by jacketing the pans or by the use of open steam.