A glue heater is nothing more than a double boiler consisting of an outer boiler, called a jacket, in which water is boiled, or through which steam passes, and an inner boiler in which the glue is prepared. Fig. 145 shows a gluepot and jacket which is used over a gas burner. In using this one must be very careful not to let the water in the outer boiler boil completely away and cause the glue to be burned. Add water when it is needed, for it boils away rapidly. When the glue reaches the right stage to be used, turn the gas under it very low, and, if the jacket is nearly full of water, it should last nearly all day without being replenished. Where practicable a steam glue heater, Fig. 146, or a contact glue heater, where the jacket comes in contact with a steam heater ring, Fig. 147, should be used for the sake of convenience and economy. The steam can be taken from the heating plant, which would be a saving of gas, With steam there is no danger of burning the glue. For our use a gluepot should have capacity for about two quarts of glue. If a steam heater is used, it should have two half gallon pots so that one pot can be kept hot while the other one is in use. Often this is a saving of much time to be able to exchange a cold pot of glue for hot glue.

Fig. 145. Glue pot and jacket.

Fig. 146. Steam glue heater.

In preparing glue the flakes are put in the gluepot and covered over with cold water and allowed to stand several hours so it will absorb a quantity of water, thus rendering it like jelly. Then it is heated until it becomes hot and thin enough to drip from the brush in a thread or stream. Care must be taken that it is not too thick nor too thin, and it must be applied hot. The gluepot must be kept clean. Care must be taken not to let the glue burn, for it burns easily if water is not kept in the outer vessel. Burning glue is very offensive and it cannot be used after it has been scorched, as its virtue is destroyed. It is not well to heat glue over and over again. After it has been heated two or three times it is well to clean the pot and make a fresh quantity, for repeated heatings tend to destroy the holding properties of the glue.

Fig. 147. Contact glue heater.

Parallel layers of wood may be glued together, but glue will not hold end woods. The surfaces which are to be fastened together must be planed true so that they fit perfectly. Glue is applied hot and quickly to each surface with a brush and then the surfaces are clamped securely together. After about twenty-four hours the clamps may be taken off. The glue being hot and thin enters the pores of the wood, getting a foothold, and when it hardens forms a joint that is stronger than the wood itself. When the gluing is well done, the wood will break in other places before it will break at the glue joint. It is well to warm the wood before the glue is applied so that the glue is not chilled before the clamps are set. This, however, is not necessary if one works fast enough and in a warm room. Of course the surfaces which are fastened together must be clean, i.e. free from dust, varnish, or anything that will not allow the glue to enter the pores of the wood. It is a good plan to clamp the pieces together to see that they fit and that the clamps work properly before the glue is applied. See that everything is ready, then apply the glue quickly and clamp before it "sets" on you.

The clamps which are used for ordinary gluing are the bar clamps, hand screws, and the iron clamps. The price of these clamps depends upon the size and kind. When two edges are to be glued together, the faces should be planed after the gluing rather than before, for then the glue which is squeezed out by the pressure of the clamps can be removed and a perfect surface obtained. Care must be taken, however, that the grain of the wood in each piece runs in the same direction.

There are many gluing exercises which can be introduced in woodwork, all of which tend to give a broader experience. Among some of these we might mention the bread board (used in cutting bread), which is made by gluing three (7/8" x l 1/2" x 12") cherry strips with two (7/8" x 1 1/2 x 12") pine strips so that the strips are arranged alternately. After gluing, the board is dressed and then cut out in the form of an ellipse. There is something very fascinating about uniting woods of different color into one piece so that the glue which holds the wood cannot be seen. Boys are delighted when they can do this work.

Another piece of work in gluing which is important is the chessboard. This is important because it teaches certain principles about building up a series of glue joints, how to prevent warping, and how to make a glue joint hold when it is necessary to glue end wood together. All of these are valuable problems to be worked out in wood construction.