screw, and, although it has been in use a long time, the present pointed screw was not made prior to the year 1841. The screw replaces nails in all fixing where the hammer cannot conveniently be used or where jarring must be avoided. The screw
Fig. 120.—Flat Head Wood Screw.
Fig. 121.—Round Head Wood Screw.
Fig. 122.—Cup Wood Screw.
possesses ten times the compression and attractive strength of ordinary nails, and, besides, is convenient for use in putting work together which is soon to be taken down. Screws are made in almost endless variety, but the best known three are: flat-head screw (Fig. 120), made of iron, steel, or brass; round-head screw (Fig. 121), which is generally japanned and used for fixing bolts, locks, etc.; cup screw (Fig. 122), the head of which fits into a cup (as illustrated) which is let into the work flush with the surface.
Glue, Glue-pots, and Glue-brushes.—Glue, size, and gelatine are varieties of the same substance, differing only in the quantity of moisture and of impurities which they contain. Gelatine-yielding substances employed in glue manufacture include skins of all animals, tendons, intestines, bladders, bones, hoofs, and horns. Glue is manufactured by boiling the animal matter and straining the product into coolers, where it thickens into a jelly, which is cut into sheets and dried in the open air on frames of wire netting. Glue should be of a bright brown or amber colour, free from specks or blotches, nearly transparent, and with but little taste or smell. It should be hard and moderately brittle, not readily affected by moisture in the atmosphere, and should break sharply, but if it shivers as easily as a piece of glass it is much too brittle, though at the same time it must not be tough and leathery. Roughly speaking, a glue which will absorb more water than another is preferable. Good glue does not give off an unpleasant smell after being prepared a few days. In the workshop, different kinds of glue-pots are used, according to the quantity required. The usual glue-pot has an outer and an inner vessel and is shown in section at Fig. 123. When glue is used in large quantities, and steam pipes are laid on for heating purposes, the glue is kept hot on a water bath heated by steam pipes. The joiner prepares glue by breaking it into small pieces, soaking these
Fig. 123.—Section through a Glue-pot.
in clean, cold water for several hours, and then boiling the resulting lumps of jelly—the superfluous water having been poured off—in a double-vessel glue-pot for an hour or two, or until the glue runs easily from the brush without breaking into drops. A glue-brush can be bought for a few pence, and its bristles should be comparatively short. A cane brush is preferred by many workers, this being made with a piece of rattan cane about 8 in. long, the flinty skin for an inch or so at one end being cut away, the end soaked in boiling water for a minute or two, and then hammered till the fibres are loosened; this brush lasts as long as there is any cane left from which to hammer out a fresh end.
Other Tools and Appliances.—Many other tools and appliances not in such general use will be illustrated and described in connection with the matter treated in some of the other sections (see index).
