USES OF GLUE With woods, glues are used to wholly or partly replace nails; and they are also used in veneered or "built-up" work, such as is called for in cabinet making, parquet floors, and the bodies of fine carriages. The latter field is so important as to warrant notice. The fact that glues are used for other purposes than to join woods has already been noted.
Veneers is applied to thin slices of wood that are later fastened to, or reinforced by, other pieces of wood. Veneers are cut with knives, or with saws. The first method, which is often preferred with more valuable woods, is compara tively economical as to material, while the latter method is easier as to labor but causes much of the log to be lost in sawdust. Rotary-cut veneers are broad ribbons pared from the surfaces of revolving logs. There are also plain-cut and quarter-cut veneers which are prepared as is indicated by their names.
Veneers ordinarily vary in thickness between one-thirtieth of an inch and one-quarter of an inch. Veneers as thin as one two-hundredth of an inch can be prepared, but the value of such sheets is lessened by the fact that the glues used to fasten them are liable to soak through and become evident on the outer surfaces. Most inside needs are met by thicknesses between one-thirtieth of an inch and one-eighth of an inch. Veneers that are to be exposed to the weather should be thicker than one-eighth of an inch. Veneers that are one-quarter of an inch or more in thickness are usually classified as thin lumber.
Preparation of Veneered Work.
The form to be covered is prepared of white pine, or some other clean, and uniformly grained wood that receives glue in a satisfactory manner. This foundation is termed the "core." Another piece, called the "caul," the surface of which coincides with that of the core, is then made ready. The pieces of veneer are fitted over the surface of the core, glued in place, and held there intimately by means of the caul. The entire series, composed of core, glue, veneer, and caul, is then placed in a press where it remains until the glue is dry. Curved, irregular, and in tricate surfaces are correspondingly harder to prepare than plain surfaces.
Door frames are made by gluing strips of wood to one another and covering them with thick veneers, or "thin lumber." Chair seats are prepared by gluing layers of wood, of equal thickness, crosswise to one another. The names three-ply or five-ply are used where three or five thicknesses of wood are thus fastened together. The roofs of many car riages are made by covering three ply roofing with heavy duck, slushed on and tacked at the edges. The use of veneers in building hulls of small boats has been alluded to.
Inlaid Work. This is prepared by fastening a sheet of light-colored wood, such as holly, upon a similar sheet of darker wood, such as mahogany. A design is traced upon the upper sheet, and a sharp knife is passed over the design so as to cut through both sheets alike. The figures cut from the lighter-tinted wood are inserted within the corresponding vacant spaces in the darker wood, or vice versa, and the sheet with insertions is glued upon a core of seasoned wood. There are many details and applica tions.
Some reasons for preferring veneered work are as follows: Stability. The natural tendency to warp and check is less when well-seasoned pieces of wood are glued crosswise to one another. Results obtained in this way are stronger, better, more rigid, and lighter in weight, than when solid wood is employed.
Appearance. The best and most beautifully figured pieces of wood are often small. Such perfect, attractive, but small pieces can be sliced, and the slices joined together so accurately, over a core of some less desirable wood, that the seams cannot easily be discovered. A large surface of perfect and uniformly beautiful wood is thus obtained.
Economy. As a matter of fact, veneered work is usually more costly than solid work of equal dimensions. It is cheaper than solid work only when the saving of material is enough to more than offset the extra expenditure for labor. From the viewpoint of material, the case is the same as when one metal is plated with another; but from the viewpoint of labor, the cost of preparing, fitting, and gluing thin sheets of wood over a curved or irregular surface, or even one that is fiat, is greater than the cost of pre paring the equivalent in solid wood.
these glues seem able to maintain very uniform products. It is said that the properties of these glues have remained constant during the last fifty years.
Samples. The selection of any sample is important. In this case, portions drawn systematically from several parts of every package should be mixed, ground, mixed again, and then divided so that the final sample will weigh about six ounces.
If glues have been mixed, the parts should be separated from one another, and each part should be tested separately. The separation of separate fragments of dissimilar glues from one another is possible because of color distinctions that usually exist. It is not enough to accept the samples presented by agents; the final deliveries also should he tested.
Appearance. Large and irregular air bubbles indicate decomposition. Smooth and glossy surfaces are desirable but not essential. The ap pearance of a glue may warrant its rejection.
Fracture. Fractures vary with moisture and, therefore, with the weather and is seldom a criterion. Similar pieces break differently at different times. Glues made from acid-treated bones generally show bright, clean fractures. .
Odor. Odor may be due to the character of the stock; or, it may be due to deterioration in the glue. Glues made from goats, sheep, oxen, and other animals, sometimes possess characteristic odors, which are often changed as a result of boiling. The odor that arises from a hot solution of glue should indicate any decomposition that has taken place after the glue has been boiled.
Acidity. The presence of acid is not an indication of the way in which the glue was manufactured, since a little acid is sometimes added after boiling. Sulphurous acid is often used to bleach glue and to pre vent its decay. A slightly acid glue is often to be preferred.
Grease. Grease is sometimes, but not always, undesirable. The properties of some glues, as those used for moulding, picture and some other purposes, may even be improved by the presence of grease. Such desirability is influenced or determined by the form in which the grease exists, which may be in large or in the form of an emul sion. Of these two forms the latter is lei objectionable. The presence of grease may be detected as follows: A few grains of some aniline color that will dissolve easily in water are placed on a sheet of clean white poper. A flat brush is dipped in a warm solution of the glue, and then hastily drained. The brush is applied to the color so as to dissolve it, and is then, without being lifted, swept across the remaining surface of the 'viper. Any grease that is present will appear in the form of spots. Some experimenters prefer to mix the aniline color with the glue at the start. The presence nee of grease in fine emulsion is not always thus revealed, since part of it may escape notice if the globules are finely divided.
Viscosity. A viscous liquid is one that moves slowly under the in fluence of any force. The molecules of such a liquid adhere to one another and do not move freely among themselves.
The degree of viscosity in a solution of glue is sometimes relied upon as an indication of strength, but is a dangerous criterion, since viscosity may be influenced by the presence of foreign substances. For example, a small quantity of alum will greatly increase viscosity without other wise improving the glue.
The degree of viscosity may be measured by noting the number of seconds required for the passage of 50 c.c. of glue solution through the orifice of a standard burette, pipette, or other vessel. Armour & Com pany test solutions prepared by dissolving one part of the glue in five parts of water, and hold the solutions thus formed at temperatures of 150 degrees Fahrenheit while they are passing through the orifices.
Foam. Minute bubbles usually appear when a solution of glue is agitated. These may shortly break and disappear, or they may remain for some time; and, in the latter case, the presence of some foreign sub stance, such as lime-soap, may be suspected. "Permanent foam" may be desirable in the case of some confectioners' gelatines, but it is not desirable in the substances now being considered.
Foam is measured in specially graduated vessels. A hot solution of glue, placed in one of these vessels, is beaten or churned for some definite time, such as half a minute. Negligible quantities of foam will shortly disappear, while larger quantities of more "permanent foam" may reach to the various graduations in the vessel.
Strength. The behavior of a piece of glue while being bent is some times taken as a criterion of its strength. It is true that a thin piece of good glue will sometimes bend almost double without breaking, and that, under the same circumstances, a sample of poor glue will sometimes break or crumble. But it is also true that some weak glues are flexible, and that some strong glues are brittle. It is not safe to estimate strength, even approximately, by the behavior of glue during bending.
A direct method of testing cements has been suggested by Ray.' Blocks of specially prepared porcelain are employed. One of the blocks resembles half of a cement briquette such as is specified for testing by the American Society of Civil Engineers, and two of the blocks, when joined together, resemble the complete form of the said briquette. The two blocks are glued together, and, after a sufficient time, are parted in a Fairbanks or other testing machine. It is hard to test glue in an ordinary wooden joint, because the glue in the joint is usually stronger than the wood. The wood fails first, and the test serves to determine the strength of the wood rather than that of the glue.
Strength is usually measured by comparing the solidity of a jelly made of the glue to be tested, with the solidity of a jelly made from glue of some standard make. The comparison may be made with a weighted plunger of some definite area; or, the jellies may be compared by pressing them, one after the other, with the finger. Fernbach, who has made many tests, regards the fourth finger of the left hand as being the most sensitive and, therefore, the most satisfactory for this work. On the whole, the jelly strength of a glue gives the best practical indica tion of its strength.
Fish Glues. Less attention has been given to perfecting methods for testing fish glues. The means that manufacturers have for grind ing these glues, and for judging their qualities, are apparently suf ficient. The fact that fish glues are sold under labels is a safeguard to consumers.