Scientific American Supplement, No. 794, March 21, 1891 by Various

V >> Various >> Scientific American Supplement, No. 794, March 21, 1891

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It was formerly supposed that plaster prepared by baking at a
temperature above 300 degrees loses completely its power of setting.
Later observations, however, as those of Landrin, negative this view.
Between 300 degrees and 400 degrees Landrin obtained plasters setting
almost instantaneously when mixed with a small amount of water. When
the temperature employed approached 400 degrees, the set plaster was
softer, but the setting still took place quickly. These observations
appear to show that the change to anhydrite is a very gradual process
at temperatures below a red heat.

Reference has been made to the differences in (1) time of setting of
plaster and (2) in hardness of the resulting material. Both of these
properties are affected by the mode of baking. The hardest material is
frequently obtained from the quick-setting plasters, but for certain
purposes this rapidity in setting is of great practical inconvenience.
Thus the moulder in pottery work must have leisure to fill in every
detail of a design often complicated and intricate before the material
with which he is working becomes intractable. Thus for many of the
more refined purposes to which plaster is applied, extreme hardness in
the set plaster is of less vital importance than a convenient period
of setting. On the other hand, plasters which set very slowly give as
a rule too soft a material, as well as being inconvenient in use.
Plasters which hit off the happy medium are alone suitable for the
work of the potter. The finer varieties of plaster prepared especially
for use in potteries are obtained by a treatment which differs in many
respects from that described above for the commoner kinds. In the
first place, the direct contact of fuel or even flame is avoided,
since this reduces some of the sulphate to sulphide of calcium, the
presence of which is in many respects objectionable. Secondly, it is
necessary that there should be a better control over the temperature,
since, as has been seen, if the heating be carried too far the
plaster, if not partially dead burnt, will set too quickly for the
particular purpose to which it is to be put.

The arrangement employed in France is known as the _four a boulanger_,
or baker's furnace. The temperature attained in the furnace itself
never exceeds low redness. The material preferred is the softer kind
of the granular variety of gypsum. This is put in in pieces of about
21/2 inches in thickness. After the baking several lumps are broken up
and examined to see that there are no shining crystalline particles,
which would indicate that some of the gypsum had remained unchanged.
Before use the plaster is ground very fine. This point is of
considerable practical importance. The consistency attained should be
such that the material may be rubbed between the finger and thumb
without any feeling of grittiness. Should there be particles of a size
to be characterized as "grit," these will after use appear at the
surface of the mould, with the result that the mould will have to be
abandoned long before it is really worn out, i.e., before the details
have lost their sharpness.

It is manifestly of considerable practical importance to understand
the conditions which determine the time of the setting up of plaster.
According to Payen, the rapidity of setting, provided the plaster has
dehydrated at a temperature sufficiently low, depends entirely on the
structure of gypsum employed. Thus, according to him, the fibrous
kinds gives a plaster setting almost instantaneously. The water, he
says, penetrates the material freely, setting takes places almost
simultaneously throughout the mass. The hydration of each particle is
accompanied by an expansion, and under the conditions specified, this
expansion being unresisted takes place to the maximum extent, with the
result of leaving cavities between the crystals, and producing a set
plaster of less coherence and density. On the other hand, where
granular crystalline gypsum has been used, setting begins at the
surface of each group of crystals before the water has penetrated to
the interior; the hydration is in consequence more gradual, and
resistance being offered to the expansion of the inner parts, a harder
and denser material is obtained. That this expansion contains an
element of truth is indicated by the practice of employing the
granular crystalline variety for the preparation of moulding plaster.
The explanation appears, however, to be inadequate in several
respects, especially in view of the fact that plasters for moulding
are reduced to a fine state of division before use. It seems as if
this treatment must, in great part at any rate, break up the
crystalline aggregates.

In order to discover a more satisfactory explanation, let us examine
the results of the chemical analysis of plasters used in commerce. One
is struck by the large percentage of water they usually contain. Thus,
four samples of ordinary plaster analyzed by Landrin have an average
of 90.17 per cent. of CaSO4 and 7.5 per cent. of water, while two
samples of best plaster contained 89.8 per cent. of CaSO4 and 7.93 per
cent. of water. These numbers do not add up to 100, the difference
being due to silica and other impurities of the original gypsum,
amounting altogether to about 3 per cent.

It might be suggested that the reason why these plasters set more
slowly than completely dehydrated plaster is owing simply to the fact
that they contain, apparently, some unaltered gypsum, which serves to
_dilute_ the action. Were this so, a similar result, as far as time of
setting is concerned, should be obtained with a plaster containing a
corresponding quantity of dead-burnt material. This, however, is not
found to be the case. The time of setting appears, then, to be
connected in some special and peculiar manner with the retention of
water by the burnt plaster.

The following explanation of this connection is offered, an
explanation only tentative at present, owing to want of experimental
data.

The following substances are known:

Gypsum, and set plaster, CaSO4 + 2 H2O, containing 20.93
per cent. of water.

Plaster completely burned at moderate temperature, CaSO4,
probably amorphous.

Anhydrite and dead-burned plaster, CaSO4, crystalline.

Selenitic deposit from boilers, 2 CaSO4 + H2O, or CaSO4 +
1/2 H2O, containing 6.2 per cent. of water.

The circumstance that the hot calcium sulphate can crystallize with 1/4
its normal amount of water indicates that for this proportion of water
it has a greater attraction than for the other 3/4. Having a similar
bearing is the fact that when burned at lower temperatures, gypsum
only loses the last portions of water with extreme slowness.

Now, if it be the case that anhydrous calcium sulphate has a greater
attraction for the first half molecule of water, then the operation of
hydration will proceed very rapidly at first, more slowly afterward.
Many such cases are known, e.g., that of copper sulphate. Conversely,
if only 3/4 of the water of hydration be expelled during the baking of
gypsum, the material obtained should hydrate itself more slowly. For
our present purpose it will be convenient to recalculate the numbers
given by Landrin (_vide supra_) so as to make the calcium sulphate and
water add up to 100. This treatment of the numbers gives a mean result
for the six analyses of 7.68 per cent. of water, the amounts not
varying by more than 1 per cent.

It will be seen that the dehydration has never passed the composition
corresponding to 2 CaSO4 + H2O; indeed, the material approximates
more nearly to the composition 3 CaSO4 + H2O. It appears probable,
therefore, that in the successful preparation of plaster the whole, or
nearly the whole, of the gypsum is changed, but that this change does
not result in the production of CaSO4, or of a mixture of CaSO4 and
CaSO4 + 2 H2O, but of a lower hydrate of calcium sulphate.

In the case of the analyses, given by Landrin, of fine plaster for
potteries, the percentages of water (8.14 and 8.08) correspond closely
to that of a hydrate, 3 CaSO4 + 2 H2O, which would contain 8.1 per
cent. of water.

Some surprise may have been excited by the fact that the well known
method of revivifying hydrated calcium sulphate has recently formed
the subject of a patent (Eng. pat., No. 15,406).

The method described in the specification consists in reducing the
materials (waste moulds, etc.) to small lumps, and baking between the
temperatures of 95 deg. and 300 deg.. It is mentioned that the whole of the
water must not be expelled. This is no doubt correct, but it must be
effected by regulating the _time_ of baking, since by prolonging the
operation all the water of crystallization can be expelled far below
300 deg.. To secure even baking the mass is kept stirred by mechanical
stirrers, a necessary precaution, since the operation is to be carried
out in an ordinary kiln. The process is stopped when a portion of the
plaster is found to set in the required time, a method of regulation
which will probably be found to work well in practice.--_Chem. Trade
Jour._

* * * * *

SPACING THE FRETS ON A BANJO NECK.

BY PROF. C.W. MACCORD.

The amateur performer on the banjo, if he be of a mechanical turn, is
often tempted to exercise his skill by making an instrument for
himself; and the temptation is the greater because he can confine
himself to the essentials. The excellence of a banjo in respect to
power and tone depends mainly upon the rim and the neck, that is,
supposing the parchment head to be of proper quality; but then the
preparation of the heads is a business of itself, and the amateur is
no more expected to make the head than to make the strings. So again,
all the minor accessories, such as pegs and tail pieces, brackets and
bridges, are kept in stock for his benefit, and he may justly claim
all the credit if his efforts in connection with the two principal
parts first mentioned result in the production of a superior
instrument. Among these ready-made items is a "fret wire" of peculiar
section, furnished with a flange ready for insertion into fine saw
cuts across the neck, which much facilitates his work.

Of course, the correctness of the notes depends entirely upon the
accuracy with which the frets are spaced, and the accompanying diagram
exhibits a convenient method of determining the spaces by graphic
means.

[Illustration: SPACING FOR BANJO FRETS]

It is to be understood that when the distance from the "nut," N, to
the bridge, B, has been determined, the first fret is to be placed at
1/18 of that distance from the nut, the distance from the first to the
second is to be 1/18 of the remainder, and so on. To determine these
distances by computation, then, is a simple enough arithmetical
exercise; but it is exceedingly tedious, since the denominators of the
fractions involved increase with great rapidity; being successive
powers of the comparatively large number 18, they soon become
enormous.

In the large diagram, the distance, A C, on the horizontal line
corresponds to the distance, N B, on the instrument. At A erect a
vertical line, and mark upon it a point B such that B C shall be
exactly eighteen times any convenient unit, B I. In the illustration B
C is 26 inches, and B I is 11/2 inches, so that B C is 27 inches in
length. About C as a center describe the arcs, B L, I K, and through I
draw a vertical line, cutting B L in D; draw the radius D C, cutting
the inner arc, I K, in J, through J draw another vertical, cutting B L
in E, and so on.

In the triangles, A B C, 1 D C, 2 E C, we have B I = D J = E F = 1/18
of the hypotenuse in each case, therefore the bases, A C, 1 C, 2 C,
are divided in the same proportion, as required, at the points 1, 2,
3. And we might extend the arcs, B L, I K, and repeat the above
operation until all the frets were located. But should that be done,
the diagram might become inconveniently large, and some of the
intersections might not be reliably determined. In order to avoid
this, the spacing of the outer arc may be stopped at any convenient
division, as L. The vertical by which that point is determined cuts B
C at B', and through B' a new arc, B' L', is described. Through the
points in which this arc cuts the radial lines already drawn, a new
series of verticals is passed, which will divide another portion of A
C as required, and by repeating this process the spacing of the whole
neck may be effected by a diagram of reasonable size.

* * * * *

GLOVE MAKING.

Glove making is almost a century old in this country, having been
begun in the neighborhood of Gloversville and Johnstown, N.Y., about
1803. Until 1862 the manufacture of gloves in Fulton County, although
even then the chief manufacturing industry, was of comparatively small
importance. Gloversville and Johnstown were then quiet villages of
from three to four thousand people. The flourishing establishments of
to-day, or such of them as then existed, were small and comparatively
unimportant. In 1862 the stimulating influence of a high protective
tariff showed itself in the increased business at Gloversville,
Johnstown, and the adjoining hamlet, Kingsboro. These became at once
the leading sources of supply for the home market gloves of a medium
grade. The quality of the product has steadily improved, and the
variety has been increased, until now American-made gloves are
steadily driving out the foreign gloves. The skill of American glovers
is equal to that of foreign glove makers, and in some respects--notably
in the quality of the stitching, and, in some grades, the shape--the
American gloves are the best. Foreign expert workmen have been drawn
over here from the great glove centers of Europe, so that the greatest
skill has been secured here. The annual value of the glove industry in
Fulton County has reached about $7,000,000.

One hundred and seventy-five glove makers and 20,000 people in Fulton
County draw their subsistence directly from glove making. Some of the
firms have a business reaching from $100,000 to $500,000 yearly. The
majority, however, have small shops, and do a small but profitable
business. Most of the work in Fulton County, as abroad, is done at the
homes of the workers. The streets of Gloversville and Johnstown are
lined with pretty and tasteful homes, in which the hum of the sewing
machine is constantly heard during the working hours of the day, but
the workers are exceptionally fortunate in being able while earning
good wages to enjoy all the comforts and surroundings of home, and in
being practically their own masters and mistresses.

Before the leather can be cut and sewed into the handsome articles
that are sold over the counters of the retail dry goods houses and
furnishing goods stores as gloves, the skins from which they are made
must be specially prepared. The two important points in this
preparation are the removal of the albuminous portion of the skin and
the retention and chemical changing of the gelatinous part, so that
it shall become pliable, elastic, and resist decomposition.

There are various methods which produce these results, and they are
technically known as tanning, alum dressing, oil dressing, and Indian
dressing. Each method produces a leather distinctly different from
that produced by any other. All the preliminary processes of these
various methods are alike in principle, although they vary somewhat in
detail. The object in all is to remove the hair from the hide,
separate the fleshy and albuminous matter, and leave only the
gelatinous, which alone is susceptible to the chemical action and can
be transformed by it into leather.

When the skins are received in the factory they are thoroughly soaked
to open out the texture and prepare them for the removal of the hair.
Then the skins are placed in vats of lime water, where, for two or
three weeks, the lime works into the flesh and albuminous matter, and
loosens the hair. The skins having thus been properly softened, the
dirty but picturesque operation of beaming for removing the hair
ensues. Before each beamer, as the workman is called, is an inclined
semi-cylindrical slab of wood covered with zinc. The skin is first
spread upon this, and the broad, curved beam of the knife glides
across it from end to end, scraping and removing all the loosened
hair, the scarf skin, and the small portion of animal matter adhering
to the skin.

After the unhairing, kid skins must be fermented in a drench of bran,
whose purpose is to completely decompose the remaining albuminous
matter, and also to remove all traces of the lime. The operation is
extremely delicate. While the gelatine is not so sensitive to the
decomposing action of the ferment, nevertheless great care is required
to prevent overfermentation and resulting damage to the texture of the
skin. It is impossible for even the most experienced to tell just how
long the fermentation should continue. Sometimes the work is done in
two or three hours, and sometimes it requires as many days. Incessant
watchfulness both day and night is required to detect the critical
moment. With the less delicate skins this bran bath is not necessary.
Lime and acid solutions accomplish the same purpose. When the gelatine
matter is all removed the skins are ready for the actual curative
process.

Oil dressing or Indian dressing--which merely differ in application,
but are founded upon the same principle--is the most simple method of
curing skins. The principle of each is the soaking of the gelatine
fibers of the skin with oil, the union of the latter and the gelatine
appearing in the form of oxide, and resulting in the insoluble,
undecomposable, pliant, and tough material known to the commercial
world as leather. The first step in the oil dressing, after the skins
have been duly soaked to render them porous and absorptive, is to
cover them with fish oil and place them in the stocks or fulling
machines--huge wooden hammers with notched faces working in iron
cases--where they are beaten and turned, and subjected to a uniform
pressure until the oil is gradually absorbed. After taking them out,
hanging them up, and stretching them, the oil and fulling process is
repeated according to the thickness of the skin, and until every part
of it is full of oil. After this the skins are dried in a mild heat
that causes the oxidization of the oil. This being completed, all the
superfluous oil is removed by putting the skins in an alkali bath.
Then the curing process is complete.

With the preparation of kid leather alum is the astringent curative
agent. Its operation is accompanied by that of others whose purpose is
to secure elasticity and pliability, and mainly to preserve that
beautiful texture which makes kid leather superior to all others.
These assistants in the process are eggs, flour, and salt. They are
combined into what is called a custard. A proper quantity of the
custard and a number of skins having been put together in a dash
wheel, where they are thrown about for some time, the open pores of
the skin absorb the custard freely, and become swelled by the chemical
union of the custard and the skin. In trade parlance this swelling is
known as "plumping." This having progressed satisfactorily, the skins
are folded together with the fleshy side outward, and are dried by a
gentle heat.

They are now cured, but they are yet hard and rough. Another
objectionable feature is that they are of unequal thickness. Breaking
and staking, as they are called, are now resorted to, to make the
skins soft, pliable, and of even texture, removing the superfluous
chemicals with which they become charged, and the stiffness by
manipulating the fibers. Much trained skill and dexterity, especially
in knee and arm staking, are required in the stretching, which is the
essential feature of these operations. Breaking is first resorted to.
The break beam, which is armed at each end with a knife edge,
oscillates up and down. In a frame beneath it the operator stretches
the dried and stiff skin. The break beam comes down upon the skin,
stretches and softens it, and removes much surplus custard. The
operator presents a new surface to each stroke of the break beam, and
in a very short space of time the entire skin is rendered soft and
pliable.

Further manipulation upon the arm or knee stake--of which a dull,
semicircular knife blade, supported upon a suitable standard upon the
floor or upon a beam about opposite the worker's elbow is the main
feature--is required. The skin must be drawn across this knife blade
with a considerable application of force so as to reduce the unduly
thick parts, stretch the skin and secure a uniform thickness suitable
for gloves. Much dexterity, especially in the case of fine skins, is
required in this operation to avoid cutting or tearing. The operator
places the fleshy side of the skin over the knife, grasps the two ends
of the skin, and placing his knee upon it and slowly drawing the skin
across the knife edge, he brings his weight to bear upon it. If the
operator is skilled and experienced the skin yields quickly, when
needed, to the strain applied and a uniform texture is secured. The
operation of transforming the skin into leather is now finished, but
age is necessary to secure perfect pliability and softness. The skins
are, therefore, laid away to let the slow chemical operation going on
within them be completed.

The visitor can now watch the further processes of manufacture by
visiting the dye rooms. Skins which have already been aged are
immersed in dye vats, where the delicate colors are imparted to them.
The same care is not required in obtaining the ordinary range of dark
colors, for these are "brushed" on, the skin being spread upon a glass
slab and the dye being painted on with a brush. After they are dyed
the skins are sometimes somewhat hard, and in some classes have to be
staked again in order to restore their pliability. The finishing
touches to a kid skin are secured by rubbing the grain side over with
a size, which imparts a gloss. The experience of Gloversville
manufacturers with "buck" gloves has enabled them to impart a special
finish to a skin which is very popular under the title of "Mocha."
This is the same as suede finish, which is produced in other countries
by shaving off the grain side of the skin at an early stage of its
progress. The Gloversville method is much better, however, and has
more perfect results. Here the grain is removed, and the velvet finish
secured by buffing the surface on an emery wheel. The surface of the
leather is cut away in minute particles by this process, and the
result is an exceedingly even and velvety texture, superior to that
obtained by other methods. European manufacturers do not approach the
Americans in this respect.

The leathermaker leaves off and the glovemaker begins.

A marble slab lies before the cutter on a table, and every particle of
dirt or other inequality is removed before "doling." The skin is
spread, flesh side up, upon the slab, and the cutter goes over it with
a broad bladed chisel or knife, shaving down inequalities and removing
all the porous portions. The dexterity with which this is done makes
the operation appear extremely simple, but any but a skilled and
experienced operative would almost surely cut through the skin. The
most delicate part of the glovemaker's art, in which exact judgment is
required, comes in preparing the "tranks" or slips, from which the
separate gloves are cut. The trank must be so cut as to have just
enough leather to make a glove of a certain size and number. The
operation would be easy enough if the material were hard and stiff,
and if the elasticity were uniform, but this is rarely the case.

To accomplish this operation the trank must be firmly stretched in one
direction, and while so stretched a "redell" stamps the proper
dimensions in the other direction, to which the leather is trimmed.
Upon the nicety with which this operation is performed depends the
question of whether the finished glove will stretch evenly or too much
or too little in one direction or the other. After this the trank or
outline of the glove must be cut out. In olden times of glove
manufacture an outline was traced upon the leather and the pattern was
cut with shears. Modern invention has produced dies and presses which
are universally used. The steel die has the outline of a double glove,
including the opening for the thumb piece. The die rests upon the bed
of the press. Several tranks are laid upon it, the lever is drawn, and
in a moment the blanks are cut out clean and smooth. The gussets,
facings, etc., are cut from the waste leather in the thumb opening at
the same operation. Similar dies are used in the cutting of the thumb
pieces and fourchettes or strips forming the sides of the fingers.

The pieces now go to the great sewing rooms of the factory, where are
long rows of busy sewing girls. If the manufacturer of years ago
could revisit the scenes of his earthly toil, and wander through the
sewing rooms of a modern factory, he would doubtless be greatly amazed
at the sight presented there. In his day such a thing was unknown. The
glove was then held in position by a hand clamp, while the sewing girl
pushed the needle in and out, making an overseam. All this is done now
in an infinitely more rapid manner by machine, and with resulting
seams that are more regular and strong than those made by the hand
sewer. The overseam sewers earn large wages, and their places are much
coveted. Overlapping seams are produced on the pique machine, which is
a most ingenious mechanism. The essential feature of this machine is a
long steel finger with a shuttle and bobbin working within, and the
finger of the glove is drawn upon this steel finger, permitting the
seam to be sewn through and through. The visitor to the factory can
see also the minor operations of embroidering, lining--in finished
gloves--sewing the facing, sewing the buttonholes, putting on the
buttons, and trimming with various kinds of thread. Before the gloves
are ready for the boxes one more operation remains. The gloves are
somewhat unsightly as they come from the sewers' hands, and must be
made trim and neat. To secure these desirable results the gloves are
taken to the "laying-off" room.

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