608

GLUE.

pegs, stuck into mortise holes in an upright, fixed round the room; so that the awr mayhave perfectly free access on every side. The cakes must moreover be turned upside do^vaupon the nets twice or thrice every day, which is readily managed, as each frame mayslid out like a drawer, upon the pegs at its two sides.

The drying of the glue is the most precarious part of the manufacture. The leasdisturbance of the weather may injure the glue during the two or three first days ofexposure ; should the temperature of the air rise considerably, the gelatine may turn sosoft as to become unshapely, and even to run through the meshes upon the pieces belo^>or it may get attached to the strings and surround them, so as not to be separabwithout plunging the net into boiling water. If frost supervene, the water may freez®and form numerous cracks in the cakes. Such pieces must be immediately re-mel teand re-formed. A slight fog even produces upon glue newly exposed a serious deterior®tion ; the damp condensed upon its surface occasioning a general mouldiness. Aderstorm sometimes destroys the coagulating power in the whole laminae at once; ores® 5the glue to turn on the nets, in the language of the manufacturer. A wind too dry ®too hot may cause it to dry so quickly, as to prevent it from contracting to its proper srwithout numerous cracks and fissures. In this predicament, the closing of all theof the windows is the only means of abating the mischief. On these accounts it I sportance to select the most temperate season of the year, such as spring and autumn? 1the glue manufacture.

After the glue is dried upon the nets it may still preserve too much flexibility, or soft®®,at least, to be saleable ; in which case it must be dried in a stove by artificial heat. *aid is peculiarly requisite in a humid climate, like that of Great Britain . . . ot

When sufficiently dry it next receives a gloss, by being dipped cake by cake gwater, and then rubbed with a brush also moistened in hot water ; after which the 0 ^is arranged upon a hurdle, and transferred to the stove room, if the weather besufficiently hot. One day of proper drought will make it ready for being packed upcasks. • '

The pale-colored, hard, and solid article, possessing a brilliant fracture, which is m , gfrom the parings of ox-hides by the first process, is the best and most cohesive, a?most suitable for joiners, cabinet-makers, painters, &c. But many workmen are in ^enced by such ignorant prejudices, that they still prefer a dark-colored article, with s° ^what of a fetid odor, indicative of its impurity and bad preparation, the result olmaterials and too long exposure to the boiling heat. . 0 f

There is a good deal of glue made in France f^om bones, freed from the phosph® ^lime by muriatic add. This is a poor article, possessing little cohesive force. 1 gj uesolves almost entirely in cold water, which is the best criterion of its imperfection. cr .should merely soften in cold water, and the more considerably it swells, the better? ©ally speaking, it is. . llC h

Some manufacturers prefer a brass to a copper pan for boiling glue, and insist ^ ^on skimming it as it boils; but the apparatus I have represented renderslittle consequence. For use, glue should be broken into small pieces, put along ^* tft ^oil-water into a vessel, allowed to soak for some hours, and subjected to the heat °* i! gta teing-water bath, but not boiled itself. The surrounding hot water keeps it long in a nfor joiners, cabinet-makers, &c. g0 lid.

Water containing only one hundredth part of good glue, forms a tremulo u - " 0 fWhen the solution, however, is heated and cooled several times, it loses the l? r0 ^ or fish-gelatinizing, even though it be enclosed in a vessel hermetically sealed. Isinglass ^glue undergoes the ‘same change. Common glue is not soluble in alcohol, bu th thatcipitated in a white, coherent, elastic mass, when its watery solution is treated ; n ati£> nfluid. By transmitting chlorine gas through a warm solution of glue a c ° l J 1 a ] c0 )iohis very readily effected, and a viscid mass is obtained like that thrown down ^ u ]phufi cA little chlorine suffices to precipitate the whole of the glue. Concentrated pgela-

acid makes glue undergo remarkable changes ; during which arft produced, sug j 0 totine, leucine, an animal matter, &c. Nitric acid, with the aid of heat, convermalic acid, oxalic acid, a fat analogous to suet, and into tannin ; so that, 111 j n iiri cone piece of skin may be made to tan another. When the mixture of gl ue ., re nd eI ^acid is much evaporated, a detonation at last takes place. Strong acetic a n o

glue first soft and transparent, and then dissolves it. Though the solatio Bi<l u .gelatinize, it preserves the property of gluing surfaces together when it ” ' ie ee« 1 }-glue dissolves a considerable quantity of lime, and also of the phosphate ot - 'j’anni®precipitated. Accordingly glue is sometimes contaminated with that salt- part ofboth natural and artificial combines with glue; and with such effect, that . p us jonglue dissolved in 5000 parts of water affords a sensible precipitate with lhel ? other asnutgalls. Tannin unites with glue in several proportions, which are to ea an0t hernumbers 1, 1$, and 2 ; one compound consists of 100 glue and 89 l ^"' se ’two S “ D '

the - . .. __

of 100 glue and 60 tannin ; and a third of 100 glue and 120 tannin.