Kitabı oku: «Handwork in Wood», sayfa 6
If the board to be measured is longer than any figure given, divide the length into two parts and add the result of the two parts obtained separately. For example, for a board 23' long and 13" wide,—take 12' × 13" =13'; add to it, 11' × 13"=11' 11"; total, 24'11".
A good general rule is to think first whether or not the problem can be done in one's head without the assistance of the square.
The table is made, as its name, Board-Measure (B. M.) implies, for measuring boards, which are commonly 1" thick. For materials more than 1" thick, multiply the B. M. of one surface by the number of inches thick the piece measures.
Fig. 196. Back of Steel Square, Essex Board Measure.
Fig. 197. Steel Square with Rafter Table.
The rafter-table is found on the back of the body of the square, Fig. 197. Auxiliary to it are the twelfth inch graduations, on the outside edges, which may represent either feet or inches.
Fig. 198. The "Run" and "Rise" of a Rafter.
By the "run" of the rafter is meant the horizontal distance when it is set in place from the end of its foot to a plumb line from the ridge end, i. e., one half the length of the building, Fig. 198. By the "rise" of the rafter is meant the perpendicular distance from the ridge end to the level of the foot of the rafter. By the pitch is meant the ratio of the rise to twice the run, i. e., to the total width of the building. In a ½ pitch, the rise equals the run, or ½ the width of the building; in a ⅓ pitch the rise is ⅓ the width of the building; in a ¾ pitch the rise is ¾ the width of the building.
To find the length of a rafter by the use of the table, first find the required pitch, at the left end of the table. Opposite this and under the graduation on the edge representing the run in feet, will be found the length of the rafter; e.g., a rafter having a run of 12' with a ¼ pitch, is 13' 5" long, one with a run of 11' and a ⅓ pitch, is 13' 28⁄12", one with a run of 7' and a ⅝ pitch, is 11' 26⁄12" long, etc.
When the run is in inches, the readings are for 1⁄12 of the run in feet: e.g., a rafter with a run of 12" and a ¼ pitch is 135⁄12", one with a run of 11" and a ⅓ pitch, is 133⁄12". Where the run is in both feet and inches, find the feet and the inches separately; and add together; e.g., a rafter with a run of 11' 6", and a ½ pitch, is 15' 68⁄12" + 86⁄12" = 16' 32⁄12".
The lumberman's board-rule, Fig. 199. To measure wood by it, note the length of the board in feet at the end of the measure. The dot nearest the width (measured in inches) gives the B. M. for lumber 1" thick.
Fig. 199. Lumberman's Board Rule.
The try-square, Fig. 200, which is most commonly used for measuring the accuracy of right angles, is also convenient for testing the width of a board at various places along its length, for making short measurements, and as a guide in laying out lines with a pencil or knife at right angles to a surface or edge. The sizes are various and are indicated by the length of the blade. A convenient size for the individual bench and for ordinary use has a blade 6" long. It is also well to have in the shop one large one with a 12" blade.
In testing the squareness of work with the try-square, care must be taken to see that the head rests firmly against the surface from which the test is made, and then slipped down till the blade touches the edge being tested, Fig. 203. The edge should be tested at a number of places in the same way: that is, it should not be slid along the piece. The try-square is also of great use in scribing lines across boards, Fig. 204. A good method is to put the point of the knife at the beginning of the desired line, slide the square, along until it touches the knife-edge; then, resting the head of the square firmly against the edge, draw the knife along, pressing it lightly against the blade, holding it perpendicularly. To prevent the knife from running away from the blade of the try-square, turn its edge slightly towards the blade.
Fig. 203. Using the Try-Square.
Fig. 204. Scribing with Knife by Try-Square.
The miter-square, Fig. 201, is a try-square fixed at an angle of 45°.
The sliding T bevel, Fig. 202, has a blade adjustable to any angle. It may be set either from a sample line, drawn on the wood, from a given line on a protractor, from drawing triangles, from the graduations on a framing square, or in other ways. It is used similarly to the T-square.
Winding-sticks, Fig. 205, consist of a pair of straight strips of exactly the same width thruout. They are used to find out whether there is any twist or "wind" in a board. This is done by placing them parallel to each other, one at one end of the board, and the other at the other end. By sighting across them, one can readily see whether the board be twisted or not, Fig. 206. The blades of two framing-squares may be used in the same manner.
Fig. 205. Winding-Sticks, 12 inches Long.
Fig. 206. Method of Using the Winding-Sticks.
Compasses or dividers, Fig. 207, consist of two legs turning on a joint, and having sharpened points. A convenient form is the wing divider which can be accurately adjusted by set-screws. A pencil can be substituted for the removable point. They are used for describing circles and arcs, for spacing, for measuring, for subdividing distances, and for scribing. In scribing a line parallel with a given outline, one leg follows the given edge, or outline, and the point of the other, marks the desired line. Used in this way they are very convenient for marking out chamfers, especially on curved edges, a sharp pencil being substituted for the steel point.
The beam-compass, Fig. 208, consists of two trammel-points running on a beam which may be made of any convenient length. It is used for describing large circles. A pencil may be attached to one point.
Calipers, outside and inside, Figs. 209, 210, are necessary for the accurate gaging of diameters, as in wood-turning.
The marking-gage, Fig. 211, consists of a head or block sliding on a beam or bar, to which it is fixed by means of a set-screw. On the face of the head is a brass shoe to keep the face from wearing. Projecting thru the beam is a steel spur or point, which should be filed to a flat, sharp edge, a little rounded and sharpened on the edge toward which the gage is to be moved, Fig. 212. It should project about ⅛" from the beam. If the spur be at all out of place, as it is likely to be, the graduations on a beam will be unreliable. Hence it is best to neglect them entirely when setting the gage and always to measure with the rule from the head to the spur, Fig. 213.
Fig. 211. Marking-Gage.
Fig. 212. Spur of Marking-Gage.
Fig. 213. Setting a Marking-Gage.
In use the beam should be tilted forward, so as to slide on its corner, Fig. 214. In this way the depth of the gage line can be regulated. Ordinarily, the finer the line the better. The head must always be kept firmly pressed against the edge of the wood so that the spur will not run or jump away from its desired course. Care should also be taken, except in rough pieces, to run gage lines no farther than is necessary for the sake of the appearance of the finished work. To secure accuracy, all gaging on the surface of wood, should be done from the "working face" or "working edge."
Fig. 214. Using the Marking-Gage.
It is sometimes advisable, as in laying out chamfers, not to mark their edges with a marking-gage, because the marks will show after the chamfer is planed off. A pencil mark should be made instead. For this purpose a pencil-gage may be made by removing the spur of a marking-gage, and boring in its place a hole to receive a pencil stub with a blunt point, or a small notch may be cut in the back end of the beam, in which a pencil point is held while the gage is worked as usual except that its position is reversed. For work requiring less care, the pencil may be held in the manner usual in writing, the middle finger serving as a guide, or a pair of pencil compasses may be used, one leg serving as a guide. A special gage is made for gaging curved lines, Fig. 215.
Fig. 215. Marking-Gage for Curves.
The cutting-gage, Fig. 216, is similar to a marking-gage, except that it has a knife-point inserted instead of a spur. It is very useful in cutting up soft, thin wood even as thick as ¼".
Fig. 216. Cutting-Gage.
The slitting-gage is used in a similar way, but is larger and has a handle.
The mortise-gage, Fig. 217, is a marking-gage with two spurs, with which two parallel lines can be drawn at once, as in laying out mortises. One form is made entirely of steel having, instead of spurs, discs with sharpened edges.
Fig. 217. Roller Mortise-Gage.
The scratch-awl, Fig. 218, has a long, slender point which is useful not only for marking lines, but for centering.
Fig. 218. Scratch-Awl.
The auger-bit-gage, Fig. 219, is a convenient tool for measuring the depth of holes bored, but for ordinary purposes a block of wood sawn to the proper length thru which a hole is bored, is a satisfactory substitute.
Fig. 219. Auger-Bit-Gage.
Screw- and wire-gages, Fig. 220, are useful in measuring the lengths and sizes of screws and wire when fitting or ordering.
Fig. 220. Screw- and Wire-Gages. a. Screw-Gage. b. Wire-Gage. c. Twist-Drill-Gage.
The spirit-level, and the plumb-line which it has largely replaced, are in constant use in carpentering, but are rarely needed in shopwork.
Blackboard compasses, triangles, etc., are convenient accessories in a woodworking classroom.
8. SHARPENING TOOLS
The grindstone for woodworking tools is best when rather fine and soft. The grinding surface should be straight and never concave. The stone should run as true as possible. It can be made true by using a piece of 1" gas pipe as a truing tool held against the stone when run dry. Power grindstones usually have truing devices attached to them, Fig. 221. A common form is a hardened steel screw, the thread of which, in working across the face of the grindstone, as they both revolve, shears off the face of the stone. The surface should always be wet when in use both to carry off the particles of stone and steel, and thus preserve the cutting quality of the stone, and to keep the tool cool, as otherwise, its temper would be drawn, which would show by its turning blue. But a grindstone should never stand in water or it would rot.
It is well to have the waste from the grindstone empty into a cisternlike box under it, Fig. 221. In this box the sediment will settle while the water overflows from it into the drain. Without such a box, the sediment will be carried into and may clog the drain. The box is to be emptied occasionally, before the sediment overflows.
Fig. 221. Power Grindstone.
In order that the tool may be ground accurately, there are various devices for holding it firmly and steadily against the stone. A good one is shown in Figs. 221 and 222. This device is constructed as follows: A board A is made 2" thick, 6" wide, and long enough when in position to reach from the floor to a point above the level of the top of the stone. It is beveled at the lower end so as to rest snugly against a cleat nailed down at the proper place on the floor. The board is held in place by a loop of iron, B, which hooks into the holes in the trough of the grindstone. In the board a series of holes (say 1" in diameter) are bored. These run parallel to the floor when the board is in place, and receive the end of the tool-holder. The tool-holder consists of four parts: (1) a strip C, 1½" thick, and as wide as the widest plane-bit to be ground. The forward end is beveled on one side; the back end is rounded to fit the holes in the main board A. Its length is determined by the distance from the edge of the tool being ground to the most convenient hole in A, into which the rear end is to be inserted. It is better to use as high a hole as convenient, so that as the grindstone wears down, the stick will still be serviceable; (2) a strip, D, of the same width as A and ⅞" thick, and 15" to 18" long; (3) a cleat, E, ⅝" × ¾", nailed across D; (4) a rectangular loop of wrought iron or brass, F, which passes around the farther end of the two strips, C and D, and is fastened loosely to D by staples or screws.
Fig. 222. Grinding Device.
Fig. 223. Holder for Grinding Chisels or Plane-Bits.
The tool to be ground slips between this loop and the strip C, and is held firmly in place by the pressure applied to the back end of D, which thus acts as a lever on the fulcrum E.
Any desired bevel may be obtained on the tool to be sharpened, by choosing the proper hole in A for the back end of C or by adjusting the tool forward or backward in the clamp. As much pressure may be put on the tool as the driving belt will stand without slipping off.
A still simpler holder for the plane-bit only, is a strip of wood 1½" thick and 2" wide, cut in the shape G shown in Fig. 223. The plane-bit fits into the saw-kerf K, and in grinding is easily held firmly in place by the hand. By inserting the rear end of the stick G into a higher or lower hole in the board A, any desired angle may be obtained. G is shown in position in Fig. 221.
All such devices necessitate a perfectly true stone. The essential features are, to have a rigid support against which the tool may be pushed by the revolving stone, to hold the tool at a fixed angle which may be adjusted, and to press the tool against the stone with considerable pressure. The wheel should revolve toward the edge which is being ground, for two reasons. It is easier to hold the tool steadily thus, and the danger of producing a wire edge is lessened. The edge as it becomes thin, tends to spring away from the stone and this tendency is aggravated if the stone revolves away from the edge. If the stone does not run true and there is a consequent danger of digging into the stone with the tool which is being sharpened, the stone would better revolve away from the edge. The grinding should continue until the ground surface reaches the cutting edge and there is no bright line left along the edge. If the grinding is continued beyond this point, nothing is gained, and a heavy wire edge will be formed.
Fig. 224. Agacite Grinder.
A very convenient and inexpensive grinding tool, Fig. 224, sold as the "Agacite grinder,"11 has a number of different shaped grinding stones made chiefly of carborundum.
The oilstone. After grinding, edge tools need whetting. This is done on the whetstone, or oilstone. The best natural stones are found near Hot Springs, Arkansas. The fine white ones are called Arkansas stones, and the coarser ones Washita stones. The latter are better for ordinary woodworking tools. The India oilstone, an artificial stone, Fig. 77, p. 58, cuts even more quickly than the natural stones. It is made in several grades of coarseness. The medium grade is recommended for ordinary shop use. Oil is used on oilstones for the same purpose as water on a grindstone. When an oilstone becomes hollow or uneven by use, it may be trued by rubbing it on a flat board covered with sharp sand, or on sandpaper tacked over a block of wood.
Slipstones, Fig. 225, are small oilstones, made into various shapes in order to fit different tools, as gouges, the bits of molding-planes, etc.
Fig. 225. Slipstone.
Files are used for sharpening saws, augers, scrapers, etc. See above, p. 90.
9. CLEANING TOOLS
The bench duster. One may be noted hanging on the bench shown in Fig. 166, p. 98. Bristle brushes for cleaning the benches are essential if the shop is to be kept tidy.
Buffer. Wherever a lathe or other convenient revolving shaft is available, a buffer made of many thicknesses of cotton cloth is very valuable for polishing tools. The addition of a little tripoli greatly facilitates the cleaning.
WOOD HAND TOOLS.—Continued
References:12
(4) Scraping Tools.
Barnard, pp. 136-142.
Wheeler, pp. 465, 473.
Griffith, pp. 71-75.
Selden, pp. 149, 177, 182.
Hodgson, I, pp. 61-74.
(5) Pounding Tools.
Barnard, pp. 24-47.
Sickels, p. 70.
Wheeler, pp. 414, 428-432.
Selden, pp. 31, 111, 156.
Goss, p. 60.
Barter, p. 128.
(6) Punching Tools.
Barnard, p. 29.
Wheeler, p. 433.
Selden, p. 161.
(7) Gripping Tools.
For holding work:
Goss, p. 63.
Wheeler, pp. 65-75, 475.
Selden, pp. 140, 147, 186, 194.
Hammacher, pp. 286-291.
For holding other tools:
Goss, pp. 56-59.
Selden, p. 143.
(8) Measuring and Marking Tools.
Goss, pp. 9-20.
Griffith, pp. 9-19.
Hodgson, The Steel Square.
Wheeler, p. 465.
Tate, pp. 21-25.
Building Trades Pocketbook, pp. 234-237.
Selden, pp. 149, 150-152, 175.
Sargent's Steel Squares.
(9) Sharpening Tools.
Barnard, pp. 136-142.
Sickels, pp. 80-85.
Wheeler, pp. 480-488.
Selden, pp. 153, 162, 172, 180.
Goss, pp. 39, 64-69.
Chapter V.
WOOD FASTENINGS
The following are the chief means by which pieces of wood are fastened together: nails, screws, bolts, plates, dowels, glue, hinges, and locks.
NAILS
Nails, Fig. 226, may be classified according to the material of which they are made; as, steel, iron, copper, and brass. Iron nails may be galvanized to protect them from rust. Copper and brass nails are used where they are subject to much danger of corrosion, as in boats.
Nails may also be classified according to the process of manufacture; as, cut nails, wrought nails, and wire nails. Cut nails are cut from a plate of metal in such a way that the width of the nail is equal to the thickness of the plate, and the length of the nail to the width of the plate. In the third dimension, the nail is wedge-shaped, thin at the point and thick at the head. Unless properly driven, such nails are likely to split the wood, but if properly driven they are very firm. In driving, the wedge should spread with and not across the grain.
Wrought nails are worked into shape from hot steel, and have little or no temper, so that they can be bent over without breaking, as when clinched. Horseshoe- and trunk-nails are of this sort. They are of the same shape as cut nails.
Wire nails are made from drawn steel wire, and are pointed, headed, and roughened by machinery. They are comparatively cheap, hold nearly if not quite as well as cut nails, which they have largely displaced, can be bent without breaking, and can be clinched.
Nails are also classified according to the shape of their heads; as, common or flat-heads, and brads or finishing nails. Flat-heads are used in ordinary work, where the heads are not to be sunk in the wood or "set."
Some nails get their names from their special uses; as, shingle-nails, trunk-nails, boat-nails, lath-nails, picture-nails, barrel-nails, etc.
The size of nails is indicated by the length in inches, and by the size of the wire for wire nails. The old nomenclature for cut nails also survives, in which certain numbers are prefixed to "penny." For example, a threepenny nail is 1¼" long, a fourpenny nail is 1½" long, a fivepenny nail is 1¾" long, a sixpenny nail is 2" long. In other words, from threepenny to tenpenny ¼" is added for each penny, but a twelvepenny nail is 3¼" long, a sixteenpenny nail is 3½" long, a twentypenny nail is 4" long. This is explained as meaning that "tenpenny" nails, for example, cost tenpence a hundred. Another explanation is that originally 1000 of such nails weighed a pound. The size of cut nails is usually still so indicated. Nails are sold by the pound.
The advantages of nails are that they are quickly and easily applied, they are strong and cheap, and the work can be separated, tho with difficulty. The disadvantages are the appearance and, in some cases, the insecurity.
The holding power of nails may be increased by driving them into the wood at other than a right angle, especially where several nails unite two pieces of wood. By driving some at one inclination and some at another, they bind the pieces of wood together with much greater force than when driven in straight.
The term brads was once confined to small finishing nails, but is now used for all finishing nails, in distinction from common or flat-headed nails. The heads are made round instead of flat so that they may be set easily with a nailset and the hole filled with a plug, or, where the wood is to be painted, with putty. They are used for interior finishing and other nice work.
Tacks, Fig. 227, vary in size and shape according to their use; as, flat-headed, gimp, round-headed, and double-pointed or matting tacks, a sort of small staple. Their size is indicated by the word "ounce." For example, a two-ounce tack is ¼" long, a three-ounce tack is ⅜" long, a four-ounce tack is 7⁄16" long, a six-ounce tack is ½" long, etc. This term once meant the number of ounces of iron required to make 1000 tacks.
Fig. 227. Tack.
Tacks are useful only in fastening to wood thin material, such as veneers, textiles, leather, matting, tin, etc. Tinner's tacks, which are used for clinching, are commonly called clinch-nails. Wire tacks, altho made, are not so successful as cut tacks because they lack a sharp point, which is essential.
Corrugated fasteners, Fig. 228, or fluted nails, are used to fasten together two pieces of wood by driving the fastener so that one-half of it will be on each side of the joint. Their size is indicated by the length and the number of corrugations, as ½", four. They are often useful where nails are impracticable.
Fig. 228. Corrugated Fastener.
Glaziers' points are small, triangular pieces of zinc, used to fasten glass into sashes.
SCREWS
(a) Wood-screws, Fig. 229, may be classified by the material of which they are made; as, steel or brass. Steel screws may be either bright,—the common finish,—blued by heat or acid to hinder rusting, tinned, or bronzed. Brass screws are essential wherever rust would be detrimental, as in boats.
(b) Screws are also classified by shape; as, flat-headed, round-headed, fillister-headed, oval-countersunk-headed, and square-headed screws. Flat-heads are most commonly used. There are also special shapes for particular purposes. Round-heads may be used either for decoration or where great drawing power is desirable. In the latter case, washers are commonly inserted under the heads to prevent them from sinking into the wood. Oval-heads are used decoratively, the head filling the countersunk hole, as with flat-heads, and projecting a trifle besides. They are much used in the interior finish of railway cars. They are suitable for the strap hinges of a chest.
The thread of the screw begins in a fine point so that it may penetrate the wood easily where no hole has been bored as is often the case in soft wood. The thread extends about two-thirds the length of the screw. Any longer thread would only weaken the screw where it most needs strength, near the head, and it does not need friction with the piece thru which it passes.
The size of screws is indicated by their length in inches, and by the diameter of the wire from which they are made, using the standard screw-gage, Fig. 220, p. 116. They vary in size from No. 0 (less than 1⁄16") to No. 30 (more than 7⁄16") in diameter, and in length from ¼" to 6".
The following is a good general rule for the use of screws: make the hole in the piece thru which the screw passes, large enough for the screw to slip thru easily. Countersink this hole enough to allow the head to sink flush with the surface. Make the hole in the piece into which the screw goes small enough for the thread of the screw to catch tight. Then all the strength exerted in driving, goes toward drawing the pieces together, not in overcoming friction. The hole must be deep enough, especially in hard wood and for brass screws, to prevent the possibility of twisting off and breaking the screw. Soap is often useful as a lubricant to facilitate the driving of screws. Where it is desirable that the heads do not show, a hole may first be bored with an auger-bit large enough to receive the head and deep enough to insert a plug of wood, which is cut out with a plug-cutter, Fig. 131, p. 84, and glued in place. If pains are taken to match the grain, the scar thus formed is inconspicuous.
In rough work, the screw may be driven into place with a hammer thru most of its length, and then a few final turns be given with a screwdriver, but this breaks the fibers of the wood and weakens their hold. In "drive-screws," Fig. 229, e, the slot is not cut all the way across the head, in order that the blows of the hammer may not close the slot.
The advantages of screws are, that they are very strong and that the work can easily be taken apart. If they loosen they can be retightened. The disadvantages are, that they are expensive, that they take time to insert, that they show very plainly, and that they do not hold well in end grain.
BOLTS
Bolts with nuts are useful where great strength is desired. There are three chief varieties, Fig. 230.
Stove-bolts are cheaply made (cast) bolts having either flat or round heads with a slot for the screwdriver, like ordinary screws.
Carriage-bolts are distinguished by having the part of the shank which is near the head, square.
Machine-bolts have square, hexagonal, or button heads.
Machine-screws, Fig. 231, are similar to stove-bolts, but are accurately cut and are measured with a screw-gage. The varieties are, a, flat-head, b, round-head, c, fillister-head, d, oval-countersunk-head, all with slots for screwdriver.
Plates, Fig. 232, include corner-irons, straight plates and panel-irons. These are made of either iron or brass and are used in fastening legs to the floor, in stiffening joints, affixing tops, etc.
Dowel-rods. Dowel-rods are cylindrical rods, from 3⁄16" to 1" in diameter, and 36", 42", and 48" long. They are commonly made of birch or maple, but maple is more satisfactory as it shrinks less and is stronger than birch.
Dowels are used as pins for joining boards edge to edge, and as a substitute for mortise-and-tenon joints.
There is, to be sure, a prejudice against dowels on the part of cabinet-makers due, possibly, to the willingness to have it appear that doweling is a device of inferior mechanics. But doweling is cheaper and quicker than tenoning, and there are many places in wood construction where it is just as satisfactory and, if properly done, just as strong. Certain parts of even the best furniture are so put together.
Shoe pegs serve well as small dowels. They are dipped in glue and driven into brad-awl holes.
Wedges are commonly used in door construction between the edges of tenons and the insides of mortises which are slightly beveled, No. 34, Fig. 266, p. 179. Or the end of a tenon may be split to receive the wedges, No. 35, Fig. 266. The blind wedge is used in the fox-tail joint, No. 36, Fig. 266.
GLUE
Glue is an inferior kind of gelatin, and is of two kinds,—animal glue and fish glue. Animal glue is made of bones and trimmings, cuttings and fleshings from hides and skins of animals. Sinews, feet, tails, snouts, ears, and horn pith are also largely used. Cattle, calves, goats, pigs, horses, and rabbits, all yield characteristic glues.
The best glue is made from hides of oxen, which are soaked in lime water until fatty or partly decayed matter is eaten out and only the glue is left. The product is cleaned, boiled down and dried.
The best and clearest bone glues are obtained by leaching the bones with dilute acid which dissolves out the lime salts and leaves the gelatinous matters. Such leached bone is sold as a glue stock, under the name of "osseine." This material together with hides, sinews, etc., has the gelatin or glue extracted by boiling again and again, just as soup stock might be boiled several times. Each extraction is called a "run." Sometimes as many as ten or fifteen runs are taken from the same kettle of stock, and each may be finished alone or mixed with other runs from other stock, resulting in a great variety of commercial glues.
Manufacturers use many tests for glue, such as the viscosity or running test, the odor, the presence of grease or of foam, rate of set, the melting-point, keeping properties, jelly strength (tested between the finger tips), water absorption (some glues absorb only once their weight, others ten or twelve times), and binding or adhesive tests. This latter varies so much with different materials that what may be good glue for one material is poor for another.
Putting all these things together, glues are classified from grade 10 to 160, 10 being the poorest. The higher standards from 60 and upwards are neutral hide glues, clear, clean, free from odor, foam, and grease. The lower standards are chiefly bone glues, used for sizing straw hats, etc. They are rigid as compared with the flexibility of hide glues. For wood joints the grade should be 70 or over. For leather, nothing less than 100 should be used, and special cements are better still.
The best glue is transparent, hard in the cake, free from spots, of an amber color, and has little or no smell. A good practical test for glue is to soak it in water till it swells and becomes jelly-like. The more it swells without dissolving the better the quality. Poor glue dissolves. Glue is sometimes bleached, becoming brownish white in color, but it is somewhat weakened thereby.
Fish glue is made from the scales and muscular tissue of fish. Isinglass is a sort of glue made from the viscera and air bladder of certain fish, as cod and sturgeon.
Liquid glue may be made either from animal or fish glue. The LePage liquid glue is made in Gloucester, Mass., one of the greatest fish markets in the country. Liquid glue is very convenient because always ready, but is not so strong as hot glue, and has an offensive odor. Liquid glues are also made by rendering ordinary glue non-gelatinizing, which can be done by several means; as, for instance, by the addition of oxalic, nitric, or hydrochloric acid to the glue solution.
To prepare hot glue, break it into small pieces, soak it in enough cold water to cover it well, until it is soft, say twelve hours, and heat in a glue-pot or double boiler, Fig. 243, p. 149. The fresher the glue is, the better, as too many heatings weaken it. When used it should be thin enough to drip from the brush in a thin stream, so that it will fill the pores of the wood and so get a grip. Two surfaces to be glued together should be as close as possible, not separated by a mass of glue. It is essential that the glue be hot and the wood warm, so that the glue may remain as liquid as possible until the surfaces are forced together. Glue holds best on side grain. End grain can be made to stick only by sizing with thin glue to stop the pores. Pieces thus sized and dried can be glued in the ordinary way, but such joints are seldom good. Surfaces of hard wood that are to be glued should first be scratched with a scratch-plane, Fig. 111, p. 79.
