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.
(=G.=) Paint brushes, or other brushes in which the bristles are set in
glue, are not suitable for use in hot glue, and those made especially
for this purpose should be purchased. For very small brushes, a strip
of basswood bark may be soaked and pounded about half an inch from the
end; these are satisfactory for small work.
=66. How to use glue.=--(=A.=) Glue should be used as hot as possible,
and of about the consistency of cream. The pieces to be glued should
be heated thoroughly and the gluing done in a warm room.
(=B.=) In factories, where it is possible, the gluing is done in a
specially fitted room which contains all necessary appliances. Vertical
and horizontal coils of steam pipes surround the room, both to furnish
heat for the room, and for the purpose of heating the material to be
glued, which should be so hot that the hand cannot rest upon it for
more than a few seconds. In a room of this sort, the temperature is
maintained at from 110° to 130° F.
(=C.=) If the best possible results are wanted, a _scratch plane_
should be used. This is a tool similar to a smoother, only its cutter
is nearly vertical, and it has teeth like a fine saw which will scratch
the wood, thus giving a better hold for the glue.
(=D.=) It is important that all clamps, handscrews, and other
appliances which are likely to be needed should be set as nearly as
possible the desired size, and so arranged as to be reached easily, for
when the glue is applied, there should not be the slightest hesitation
or delay in getting the work together and the clamps on. The utmost
speed and surety of motion is absolutely necessary in using hot glue;
therefore everything during the process of the work should be foreseen
and provision made for it before the glue is applied, for if the glue
is even slightly chilled, the work will not be so well done, and the
efficiency of the glue will be greatly diminished. A novice should
never attempt more than the simplest work, unless working with a
competent man.
(=E.=) The glue should be spread rapidly and evenly with a brush of
suitable size,--a large one for broad surfaces and a small one for
small work. Glue should not be thrown about wastefully; enough should
be used to cover the surface completely but not thickly.
(=F.=) In using handscrews, it is of the greatest importance that the
jaws be kept parallel as described in Topic =59=; care must be used
that more strain is not placed upon the handscrews and clamps than is
necessary to bring the joint together.
(=G.=) Cold or liquid glue has supplanted hot glue in furniture
repairing, gluing up intricate work, and in places where it is
impracticable to use hot glue either on account of its setting too
rapidly, or where heat is not available. Liquid glue does not hold as
well nor as permanently as hot glue when properly used, but for many
kinds of work it is perfectly satisfactory.
(=H.=) In gluing rosewood, or other woods of a greasy nature, the glue
should be thinned with vinegar, which will cut the grease. Another
method of making glue hold on wood of this sort is to chalk both
members of the joint thoroughly, and let it stand for two or three
hours, when it should be wiped off. This absorbs the grease on the
surface of the wood, which allows the glue to take hold. In all gluing,
do not allow the bare hand to touch the joint any more than necessary,
as the grease and perspiration will prevent the best results from being
obtained.
=67. The testing of sandpaper.=--(=A.=) Sandpaper is made by covering
paper with a thin layer of glue, over which is spread evenly a layer
of ground flint or glass; over this is spread another coating of glue,
which firmly fastens the sand to the paper.
(=B.=) In buying sandpaper, pass the finger over it to see if the
sand is firmly fastened. Be sure that the paper is neither flimsy nor
brittle. Coarse particles of sand are sometimes found upon sandpaper
which renders it worthless; these can be detected only by use, unless
they are very prominent.
=68. How to use sandpaper.=--(=A.=) Sandpaper is made in numbers,
00, 0, ½, 1, 1½, 2, 2½, 3. Numbers 00 and 0 are very fine, and are
used in rubbing down shellac and varnish. Numbers ½ and 1 are used in
sandpapering mahogany and other fancy woods, and number 1½ is used upon
all building finish but the finest; the coarser numbers are used upon
floors, outside finish, and other coarse work which is to be painted,
though for a very nice floor, 1½ is used, rarely anything finer. It is
a fallacy to think that the finer the sandpaper used, the finer the
job will be, since upon some kinds of woods fine sandpaper will make a
glassy surface in spots which will not take the finish like the rest of
the work. Sandpapering is as apt to detract from the work as it is to
improve it, for unless used very skillfully, the character of angles
and small surfaces will be changed, though it may seem that the damage
is so slight as to be imperceptible. In using sandpaper, the workman
should guard against rounding off square corners or destroying the form
of surfaces; a raw corner, however, should be removed with a few light,
careful strokes, as a perfectly sharp corner will always be more or
less ragged.
_The one who knows_ will always notice the omissions of details of
this sort, and will attribute such imperfections to lack of skill or
knowledge on the part of the workman. It is for the one who knows,
that all work should be done--not for the casual observer--and these
apparently insignificant details, rather than the part of the work
which may seem of more importance, form the basis by which one workman
judges the work of another. In nothing do small things count more than
in making or destroying a workman’s reputation.
[Illustration: FIG. 97.--USE OF SANDPAPER UPON A BROAD SURFACE.]
(=B.=) Keep the sandpaper dry, and stored in a dry place, as moisture
softens the glue so that the sand may be easily rubbed off. In handling
sandpaper, care should be taken that the sanded sides are not rubbed
together.
(=C.=) In preparing to sandpaper a flat surface, or for general work, a
sheet of sandpaper should be torn in halves the short way of the paper,
and one half should be folded back to back, and held (not tacked)
around the block with the hand, as in Fig. 97. The act of grasping the
block for the work will hold the sandpaper, and any device for holding
the sandpaper on the block is worthless, being considered by the
workman as a mark of the novice. The block should be about 3” × 4” ×
⅞”, and may be made of wood, cork, fiber, or any material which suits
the taste of the workman.
If there is much sandpapering of moldings to be done, it is best to
make blocks which will fit the contour of them, as it is very hard on
the hands to do this work for very long at a time, though nothing has
ever been invented which fits irregular forms as well as the fingers. A
piece of sandpaper _should never be used on a piece of work until all
the cutting by edge tools has been done_, as the particles of sand will
enter the grain of the wood, and any edge tools used upon it afterward
will be quickly dulled. Do not use a piece of sandpaper so large that
any part of it will not be under perfect control, as loose ends will
scratch the wood, and it has an awkward and unworkmanlike appearance.
Always work parallel with the grain, and be sure that all plane marks
and rough places are thoroughly rubbed down. In order to do this well,
it is often necessary to use considerable muscle. This part of the work
calls for good judgment, for unless sandpapered enough, there will be
places which will show when the finish is spread on the work, though
they may have been invisible before. No one can tell as well as the
workman himself when sufficient sandpapering has been done, though it
may be evident to any one who knows the signs whether or not the work
has been done judiciously. Upon a coarse job it is usually allowable,
and sometimes desirable, to sandpaper across the grain, especially if
the work is to be painted.
In order to impress it upon the student, we will repeat that _too much
care cannot be taken in the use of sandpaper_, for much oftener will an
amateur injure a piece of work than improve it.
[Illustration: FIG. 98.--SANDPAPERING PANEL WORK.
(For explanation, see text.)]
(=D.=) In sandpapering panel work, as in Fig. 98, the panels (_a_)
should be smoothed, scraped, and sandpapered, and the edges of the
stiles (_b_), rails (_d_), and muntins (_e_) should be treated the
same way before the panel work is put together; an exception to this
in regard to the panels may be made if the panel frame is constructed
in such a way as to allow the panels to be put in place after it is
together, in which case the panels may be smoothed at any time. Upon
very fine work the panels are sometimes polished before being put in
place, as it is difficult for the finisher to work into the corners
after the panels are in place. After the faces of the stiles, rails,
and muntins have been planed and scraped, they should be sandpapered
in the order named, working with the sandpaper over a sharp-cornered
block close to the edges of the pieces, being careful not to drag the
paper over the face of the pieces which join at right angles. The
stiles, rails, and muntins should be sandpapered in the order in which
they are mentioned. If the sandpaper runs over the rails a little when
sandpapering the muntins, or over the stiles when sanding the rails, it
will do no harm, as a couple of light, careful strokes parallel with
the grain will be sufficient to remove any scratches which may be made.
In sanding mahogany, or any wood of which the grain rubs up, make the
strokes in one direction only, instead of back and forth. Sometimes wax
is rubbed in to hold the grain down upon cheap work, but this is not
recommended, as that place will not take the stain or the finish like
the rest of the wood. A very thin coat of shellac is used for the same
purpose; this is less objectionable, but should be avoided if possible.
SUGGESTIVE EXERCISES
65. Of what material is glue made? Describe briefly the process of
making glue. What kind of glue is best for general work? Is ground
glue always reliable? What is the chief advantage in its use? Is
high-priced glue always the best for all purposes? What should be the
appearance of good glue? How should it act in cold water? When cut
with a knife? When broken? Compare the amount of water absorbed by a
good and a poor glue. How should a glue pot be constructed? What will
be the result if the glue pot boils dry? Describe brushes suitable for
use in gluing. What kind of bark makes a good brush for small work?
How is it prepared for use?
66. Describe the condition of glue when ready for use. What tool is
used to increase the strength of the joint? In preparing for gluing,
what preparations should be made? How should wood be treated for use
in gluing up wood of a greasy nature?
67. Describe the manufacture of sandpaper. What is used for sand? How
select sandpaper?
68. For what kind of work is sandpaper numbers 00 and 0 used? Numbers
1/2 and 1? What number of sandpaper is used upon general work? What
will be the result if sandpaper is kept in a damp place, or becomes
wet? What should be the size of the piece of sandpaper used upon flat
surfaces, and for general work? How should moldings be sandpapered?
What should be guarded against in working around sharp corners? Should
sandpaper be carried with or across the grain? What exceptions? How
should panel work be sandpapered? How should panels and the edges of
stiles, rails, and muntins be treated before gluing up? How should
sandpaper be used upon grain which rubs up? How are panels sometimes
treated upon fine work? Why?