Elements of woodwork

by Charles A. King

Glue and Sandpaper

65. Different kinds of glue.—(A.) Wood-workers use both liquid and sheet or stick glue, but as the former requires little skill in its use, we will deal principally with the latter, which is made of hides, sinews, bones, and waste material of slaughterhouses. Different grades of glue are made of various kinds of refuse, but the processes of treating them all are similar.

(B.) The material from which glue is to be made is steeped in lime water at low temperature, or subjected to a chemical treatment for a sufficient time to separate the fat from the fiber. The latter is then washed in clean water and boiled down to gelatin, which is spread upon wires to dry and harden, when it is ready for use.

(C.) Ground glue makes up more readily than that which comes in sheets, and therefore is preferred by many workmen. It is frequently adulterated, but if made upon honor, it is as good as the glue from which it is made, and does not deteriorate unless kept for a long time in a damp place. As it is not possible to apply certain tests to ground glue which may be used upon that in sheets or sticks, many workmen prefer not to use it unless sure of its quality.

(D.) The highest-priced glue is not always the best for all purposes, and a dealer who handles different grades can generally advise which should be used, though the medium grade in common use is usually satisfactory for general work.

(E.) It is impossible to give infallible rules for testing glue in the stick, or to say that glue should be of any special color, or that it should be either transparent or opaque; but, in general, glue suitable for ordinary work will be of a reddish, yellowish, or light brownish color, clear and transparent, and not offensive to either taste or smell, though some of the best makes of glue are absolutely opaque. Good glue will swell in cold water, but will not dissolve until it has nearly reached the boiling point. It will also absorb more water than will poor glue, and is therefore more economical. Any test which depends upon the brittleness or dryness of the glue is not reliable, as a somewhat damp, good glue will not stand this test as well as a poor glue that is very dry. If conditions are the same, and comparison is possible, it is fairly safe to assume that if a good glue is cut with a sharp knife, a hard, elastic shaving will result, while a poor glue will give a shaving which is extremely brittle, and will break into little pieces.

A safe way to test glue is to prepare a number of pieces of the same kind of wood, 1” square and about 12” long, fit them perfectly end to end in pairs, and glue as many of them together as there are samples of glue to be tested. After the glue is thoroughly hard, clamp one of the pieces of each pair to a bench top, with the joint coinciding with the edge. Hang a pail about 10” from the joint on the piece which projects over the edge of the bench, and allow sand to run into it slowly, until the joint breaks. Repeat this process with each pair which has been glued up, and the amount of sand necessary to break the joint will furnish a basis of comparison between the different varieties of glue tested.

(F.) Glue should be soaked in cold or lukewarm water before being put into a glue pot, which should be a double vessel, with the glue in the inside pot, and the hot water or steam in the outer jacket. In making up glue, it should be brought to the boiling point until melted, and then removed from the heat, for if kept continually hot, it loses much of its strength by being cooked too much, as this makes continual thinning necessary.

If time will not permit, the preliminary soaking may be dispensed with, and the hard glue put at once in the hot water, in which case it must be stirred frequently while melting, or it will form a mass. If the water boils out of the outside kettle, and the glue burns, throw it away, as it is worthless.

Glue should be thinned with cold water, after which it should be allowed to become thoroughly heated before using; in the shops, this is not always done, as there may not be time to allow the glue to become heated again; therefore it is quite the common custom to thin the glue with hot water.