shape in copper, it is passed on to another class of artisans, who
prepare it for the hands of the enameler. The design or designs are
sketched carefully. The working appliances consist only of a pointed
tool, two or three small punches of varying sizes, and a hammer. With
this small equipment the operator sets to work. The spaces between each
dividing line are gradually lowered by hammering, and when this has
been uniformly completed, each little recess is ready to receive its
allotment of enamel. More accurate work even than this can be obtained
by the introduction of flat wire. This wire is soldered or fixed on the
vase, and forms the outline for the entire design. It may be of brass,
copper, or gold, but is fixed and built round every item of the whole
design with the most {295} laborious care. It stands above the surface
of the design on the copper articles, but the little recesses formed
by it are then gradually filled up by enamel in successive fusings.
The whole surface of the article is now ground perfectly smooth and
polished until its luster is raised to the highest point possible, and
when this stage has been reached the article is ready for the market.
«From the Sheet to the Sign Tablet.»—The plates are generally in
lengths of 6 feet by 2 feet, 6 feet by 3 feet, etc., the gauge
generally being from 14 to 22, according to the size and class of
plates to be enameled. These must be cut, but some enamelers prefer
to order their plates in specified sizes, which does away with the
necessity of cutting at the enameling factory. In order, however, to
make this article complete, we will assume that a stock of large plates
is kept on hand, the sizes being 6 feet by 3 feet and 6 feet by 2
feet. An order for sign tablets is given; particulars, say as follows:
Length, 2 feet by 12 inches, white letters on blue ground; lettering,
The Engineer, 33 Norfolk Street; block letters, no border line, 2
holes. For ordinary purposes these particulars would be sufficient for
the enameler.
Stage I.—Cutting the plate is the first operation. The plates 6 feet
by 2 feet would first be cut down the center in a circular cutting
machine, thus forming two strips, 6 feet by 12 inches. Each strip would
then be cut into three lengths of 2 feet each. If a guillotine had to
be used instead of a circular cutter, the plate would be first cut
transversely at distances of 2 feet, thus forming three square pieces
of 2 feet by 2 feet. These would then be subdivided longitudinally into
two lengths each, the pieces being then 2 feet by 12 inches. Each sheet
would thus be cut into six plates.
Stage II.—The cut plates should next have any roughness removed from
the edges, then punched with two holes—one at each end, followed by
leveling or setting. This is done by hammering carefully on a true flat
surface.
Stage III.—The plates should then be taken and dipped into a
hydrochloric acid bath made up of equal quantities of the acid and
water. The plates are then raised to a red heat in the stoves, and on
removal it will be found that the scale—iron oxide—has become loosened,
and will readily fall off, leaving a clean metallic surface. A second
course of cleaning then follows in diluted sulphuric acid—1 part acid
to 20 parts water. In this bath the iron may be kept for about 12
hours. In some cases a much stronger bath is used, and the plates are
left in only a very short time. The bath is constructed of hard wood
coated inside with suitable varnish.
In mixing the sulphuric acid bath it must be remembered that the acid
should be slowly poured into the water under continuous stirring.
Following the bath, the metal is rinsed in water, after which it is
thoroughly scoured with fine flinty sand. Rinsing again follows, but
in boiling water, and then the metal is allowed to dry. The enameling
process should immediately follow the drying, for if kept for any
length of time the surface of the metal again becomes oxidized. In
hollow-ware enameling the hydrochloric acid bath may be omitted.
Stage IV.—The plates are now ready for the reception of the foundation
or gray coating. If powder is used the plate is wiped over with a gum
solution, and then the powder is carefully and uniformly dusted through
a fine sieve over the surface. The plate is then reversed and the
operation repeated on the other side. If a liquid “gray” is to be used
it should have a consistency of cream, and be poured or brushed with
equal care over the two surfaces in succession, after the plate has
been heated to be only just bearable to the touch. The plates are then
put on rests, or petits, in a drying stove heated to about 160° F.,
and when thoroughly dry they are ready for the fusing operation. The
petits, with the plates, are placed on a long fork fixed on a wagon,
which can be moved backward and forward on rails; the door of the
fusing oven is then raised and the wagon moved forward. The fork enters
the oven just above fire clay brick supports arranged to receive the
petits. The fork is then withdrawn and the door closed. The stove has a
cherry-red, almost white, heat and in a few minutes the enamel coating
has been uniformly melted, and the plates are ready to be removed on
the petits and fork in the same manner as they were inserted. Rapid
cooling must now be carefully avoided, otherwise the enamel and
the iron will be liable to separate, and chipping will result. The
temperature of fusion should be about 2,192° F.† When all the plates
have been thus prepared they are carefully examined and defective ones
laid aside, the others being now ready for the next operation. {296}
────────────────────────────
† Melting a piece of copper will approximately represent this
temperature.
Stage V.—The coating of the plate with white is the next stage. The
temperature of fusion of the white glaze is lower than that of the
gray, so that the plate will remain a shorter time in the stove, or
be submitted to a somewhat lower temperature. The latter system is to
be strongly recommended in order to prevent any possibility of fusion
of the ground mass. The white should be made as liquid as possible
consistent with good results. The advantages of thin coatings have
already been explained, but if the mixing is too thin the ground
coating will not only be irregularly covered. but, in fusion, bubbles
will be produced, owing to the steam escaping, and these are fatal
to the sale of any kind of enameled ware. When the plate has been
thoroughly dried and fusion has taken place, slow and steady cooling
is absolutely essential. Special muffles are frequently built for this
purpose, and their use is the means of preventing a large number of
wasters. Before putting on the glaze, care must be taken to remove the
gray from any part which is not to be coated. The temperature of fusion
should be about 1,890° F.,† and the time taken is about 5 minutes.
────────────────────────────
† Melting a piece of brass will represent this temperature.
Stage VI.—The stencil must be cut with perfect exactitude. The letters
should be as clear as possible, proportioned, and spaced to obtain the
best effects as regards boldness and appearance. Stencils may be cut
either from paper or from specially prepared soft metal, called stencil
metal. The former are satisfactory enough when only a few plates are
required from one stencil, but when large quantities are required, say,
60 upward, metal stencils should be used. The paper should be thick,
tough, and strong, and is prepared in the following manner: Shellac
is dissolved in methylated spirits to the ordinary liquid gum form,
and this is spread over both sides of the paper with a brush. When
thoroughly dry a second protective coating is added, and the paper is
then ready for stencil work. The stencil cutter’s outfit consists of
suitable knives, steel rule, scales of various fractions to an inch, a
large sheet of glass on which the cutting is done, and alphabets and
numerals of various characters and types. For ordinary lettering one
stencil is enough, but for more intricate designs 2, 3, and even 4
stencils may be required. In the preparation of the plates referred to
in the paragraph preceding Stage I, only 1 stencil would be necessary.
The paper before preparation would be measured out to the exact size of
the plate, and the letters would be drawn in. The cutting would then
be done, and the result shown at Fig. 1 would be obtained, the black
parts being cut out. The lines or corners of each letter or figure
should be perfectly clear and clean, for any flaw in the stencil will
be reproduced on the plate.
[Illustration: Fig. 1]
[Illustration: Fig. 2]
Stage VII.—The next stage is the application of the blue enamel. The
operation is almost identical with that of the white, but when the
coating has been applied and dried, the lettering must be brushed out
before it is fused. The coating is generally applied by a badger brush
after a little gum water has been added; the effect of this is to make
the blue more compact.
Stage VIII.—The next operation is brushing; the stencil is carefully
placed over the plate, and held in position, and with a small hand
brush with hard bristles the stencil is brushed over. This brushing
removes all the blue coating, which shows the lettering and leaves
the rest of the white intact. When this has been done, the stencil
is removed and the connecting ribs of the lettering—some of which are
marked X in Fig. 2—are then removed by hand, the instrument generally
being a pointed stick of box or other similar wood.
Stage IX.—Fusing follows as in the case of the white glaze, and the
plate is complete. One coat of blue should be sufficient, but if any
defects are apparent a second layer is necessary.
The white and blue glazes are applied only on the front side of the
plate, the back side being left coated with gray only.
«From the Sheet to the Hollow Ware.»—In hollow-ware enameling, the
iron is received in squares, circles, or oblongs, of the size required
for the ware to be turned out. It is soft and ductile, and by means
of suitable punches and dies it is driven in a stamping press to the
necessary shape. For shallow articles only one operation is necessary,
but for deeper articles from 2 to 6 operations may be {297} required,
annealing in a specially constructed furnace taking place between each.
Following the “drawing” operations comes that of trimming; this may be
done in a press or spinning lathe, the object being to trim the edges
and remove all roughness. The articles are now ready for enameling.
For explanation, let us suppose they are tumblers, to be white inside,
and blue outside. The gray is first laid on, then the white, and
lastly the blue—that is, after the pickling and cleaning operations
have been performed. The line of demarcation between the blue and
white must be clear, otherwise the appearance of the article will not
be satisfactory. The process of enameling is exactly the same as for
sign-plate enameling, but more care must be exercised in order to
obtain a smoother surface. While the liquid enamels are being applied,
circular articles should be steadily rotated in order to let the
coating flow uniformly and prevent thick and thin places. The enameling
of “whole drawn” ironware presents no difficulty to the ordinary
enameler, but with articles which are seamed or riveted, special care
and experience is necessary.
Seamed or riveted parts are, of course, thicker than the ordinary
plate, will expand and contract differently, will take longer to
heat and longer to cool, and the conclusion, therefore, that must
be arrived at is that the thickness should be reduced as much as
possible, and the joints be made as smooth as possible. Unless special
precautions are taken, cracks will be seen on articles of this kind
running in straight lines from the rivets or seams. To avoid these, the
enamel liquid must be reduced to the greatest stage of liquidity, the
heat must be raised slowly, and in cooling the articles should pass
through, say, 2 or 3 muffles, each one having a lower temperature than
the preceding one. It is now generally conceded that the slower and
more uniform the cooling process, the greater will be the durability
of the enamel. Feldspar is an almost absolutely necessary addition to
the gray in successful hollow-ware enameling, and the compositions of
both gray and white should be such as to demand a high temperature for
fusion. The utensils with the gray coating should first be raised to
almost a red heat in a muffle, and then placed in a furnace raised to
a white heat. The white should be treated similarly, and in this way
the time taken for complete fusion at the last stage will be about 4
minutes.
The outside enamel on utensils is less viscous than the inside enamel,
and should also be applied as thinly as possible.
«Stoves and Furnaces.»—Fritting and Fusing.—The best results are
obtained in enameling when the thoroughly ground and mixed constituents
are fused together, reground, and then applied to the metal surface. In
cheap enamels the gray is sometimes applied without being previously
melted, but it lacks the durability which is obtained by thorough
fusion and regrinding. In smelting enamel one of two kinds of furnaces
may be used, viz., tank or crucible. The former is better adapted to
the melting of considerable quantities of ordinary enamel, while the
latter is more suitable for smaller quantities or for finer enamels as
the mixture is protected from the direct action of the flames by covers
on the crucibles. The number of tanks and crucibles in connection
with each furnace depends upon the heating capacity of the furnace
and upon the out-turn required. They are so arranged that all or any
of them can be used or put out of use readily by means of valves and
dampers. Generally, they are arranged in groups of from 6 to 12, placed
in a straight or circular line, but the object aimed at is complete
combustion of the fuel, and the utilization of the heat to the fullest
extent. One arrangement is to have the flame pass along the bottom and
sides of the tank and then over the top to the chimney.
The general system in use is, however, the crucible system. The
crucibles are made from the best fire clay, and the most satisfactory
are sold under the name of “Hessian crucibles.” The chief objection to
the use of the crucibles is that of cost. They are expensive, and in
many factories the life of the crucible is very short, in some cases
not extending beyond one period of fusion. When this, however, is the
rule rather than the exception, the results are due to carelessness.
Sudden heating or cooling of the crucible will cause it to crack or
fall to pieces, but for this there is no excuse. Running the molten
material quickly out of the crucible and replacing it hurriedly with
a fresh cold mixing is liable—in fact, almost certain—to produce
fracture, not only causing the destruction of the crucible, but also
the loss of the mixing. New crucibles should be thoroughly dried in a
gentle heat for some days and then gradually raised to the requisite
temperature which they {298} must sustain for the purposes of fusion.
Sometimes unglazed porcelain crucibles specially prepared with a large
proportion of china clay are used. These are, however, expensive and
require special attention during the first melt. The life of all
crucibles can be lengthened by: (1) Gradually heating them before
putting them into the fire; (2) never replacing a frit with a cold
mass for the succeeding one; it should first be heated in a stove and
then introduced into the crucible; (3) carefully protecting the hot
crucibles from cold draughts or rapid cooling.
[Illustration: Fig. 3]
«Melting and Melting Furnaces.»—The arrangement of the melting furnace
must be such as to protect the whole of the crucible from chills. The
usual pit furnaces, with slight modifications, are suitable for this
purpose. The crucible shown at _b_ in Fig. 3 is of the type already
described; at the top it is fitted with a lid, _a_, hinged at the
middle, and at the bottom it is pierced by a 2-inch conical hole.†
The hole, while melting is going on, is plugged up with a specially
prepared stopper. The crucible stands on a tubular fireproof support,
_c_, which allows the molten mass to be easily run off into a tub of
water, which is placed in the chamber, _d_. The fuel is thrown in from
the top, and the supply must be kept uniform. From 4 to 6 of these
furnaces are connected with the same chimney; but before passing to
the chimney the hot gases are in some cases used for heating purposes
in connection with the drying stove. The plug used may be either a
permanent iron one coated with a very hard enamel or made from a
composition of quartz powder and water. An uncovered iron plug would be
unsuitable owing to the action of the iron on the ingredients of the
mixing.
────────────────────────────
† Two inches for gray, one inch for glaze; the hole should he wider at
the top.
In some cases only a very small hole is made in the crucible and no
stopper used, the fusion of the mixing automatically closing up the
hole. In some other factories no hole is made in the crucible, and
when fusion is complete the crucible is removed and the mixing poured
out. The two latter systems are bad; in the first there is always some
waste of material through leakage, and in the latter the operation of
removing the crucible is clumsy and difficult, while the exposure to
the colder atmosphere frequently causes rupture.
The plug used should be connected with a rod, as shown in Fig. 3,
which passes through a slot in one-half of the hinged lid, _a_. When
fusion is complete this half is turned over, and the plug pulled up,
thus allowing the molten mass to fall through into the vat of water
placed underneath. The mixing in the crucibles, as it becomes molten,
settles down, and more material can then be added until the crucible
is nearly full. If the mixing is correctly composed, and has been
thoroughly fused, it should flow freely from the crucible when the plug
is withdrawn. Fusing generally requires only to be done once, but for
fine enamels the operation may be repeated. The running off into the
water is necessary in order to make the mass brittle and easy to grind.
If this was not done it would again form into hard flinty lumps and
require much time and labor to reduce to a powder.
A careful record should be kept of the loss in weight of the dried
material at each operation. The weighings should be made at the
following points: (1) Before and after melting; (2) after crushing.
The time required for melting varies greatly, but from 6 to 9 hours
may be considered as the extreme limits. Gas is much used for raising
the necessary heat for melting. The generator may be {299} placed in
any convenient position, but a very good system is to have it in the
center of a battery of muffles, any or all of which can be brought into
use. When quartz stoppers are used there is considerable trouble in
their preparation, and as each new batch of material requires a fresh
stopper, wrought-iron stoppers have been introduced in many factories.
These are coated with an enamel requiring a much higher temperature of
fusion than the fundamental substance, and this coating prevents the
iron having any injurious action on the frit.
«Fusing.»—For fusing the enamel muffle furnaces are used; these
furnaces are simple in construction, being designed specially for:
(1) Minimum consumption of fuel; (2) maximum heat in the muffle; (3)
protection of the inside of the muffle from dust, draughts, etc.
The muffle furnaces may be of any size, but in order to economize fuel,
it is obvious that they should be no larger than is necessary for the
class and quantity of work being turned out. For sign-plate enameling
the interior of the muffle may be as much as 10 feet by 5 feet wide
by 3 feet in height, but a furnace of this kind would be absolutely
ruinous for a concern where only about a dozen small hollow-ware
articles were enameled at a time. The best system is to have 2 or 3
muffle furnaces of different dimensions, as in this way all or any one
of them can be brought into use as the character and number of the
articles may require. The temperature throughout the muffle is not
uniform, the end next to the furnace being hotter than that next to
the door. In plate enameling it is therefore necessary that the plates
should be turned so that uniform fusion of the enamel may take place.
In the working of hollow ware the articles should be first placed at
the front of the muffle and then moved toward the back. The front of
the furnace is closed in by a vertically sliding door or lid, and in
this an aperture is cut, through which the process of fusion can be
inspected. All openings to the muffle should be used as little as
possible; otherwise cold air is admitted, and the inside temperature
rapidly lowered.
[Illustration: Fig. 4]
Fig. 4 shows a simple arrangement of a muffle furnace; _a_ is the
furnace itself, with an opening, _e_, through which the fuel is fed;
_b_ is the muffle; _c_ shows the firebars, and _d_ the cinder box; _f_
is a rest or plate on which is placed the articles to be enameled. The
plate or petits on which the articles rest while being put into the
muffle should be almost red hot, as the whole heat of the muffle in
this way begins to act immediately on the enamel coating. The articles
inside the muffles can be moved about when necessary, either by a hook
or a pair of tongs, but care must be taken that every part of the
vessel or plate is submitted to the same amount of heat.
[Illustration: Fig. 5]
In Figs. 5, 6, and 7 are given drawings of an arrangement of furnaces,
etc., connected with an enameling factory at {300} present working.
The stoves shown in Fig. 5 are drying stoves fired from the end by
charcoal, and having a temperature of about 160° F. Fig. 6 shows the
arrangement of the flues for the passage of the gases round the fusing
oven. The section through the line _A B_, Fig. 5, as shown in Fig.
7, and the section through the frit kilns, as shown in Fig. 8, are
sufficiently explanatory. The frit kilns and the fusing oven flues
both lead to the brick chimney, but the stoves are connected to a
wrought-iron chimney shown in Fig. 6. Another arrangement would have
been to so arrange the stoves that the gases from the frit kilns could
have been utilized for heating purposes.
[Illustration: SECTION THROUGH FUSING OVEN
Fig. 6]
[Illustration: SECTION ON A. B.
Fig. 7]
[Illustration: SECTION THROUGH FRIT KILNS
Fig. 8]
«Fuel.»—The consumption of fuel in an enameling factory is the most
serious item of the expenditure. Ill-constructed or badly proportioned
stoves may represent any loss of coal from a quarter to one ton per
day, and as great and uniform temperatures must be maintained, fuel of
low quality and price is not desirable. In the melting stoves either
arranged as tank or crucible furnaces, the character of the coal must
not be neglected, as light dust, iron oxide, or injurious gases will
enter into the crucibles through any opening, especially if the draught
is not very great. Almost any of the various kinds of fuel may be used,
provided that the system of combustion is specially arranged for in
the construction of the furnaces. Charcoal is one of the best fuels
available, its calorific value being so great; but its cost is in some
places almost prohibitive. Wood burns too quickly, and is therefore
expensive, and necessitates incessant firing.
For practical purposes we are thus often left to a selection of some
type of coal. A coal with comparatively little heating power at a
cheap price will be found more expensive in the end than one costing
more, but capable of more rapid combustion and possessing more heat
yielding gases. Cheap and hard coals give the fireman an amount of
labor which is excessive. The proper maintenance of the temperature
of the stove is almost impossible. Anthracite is excellent in every
way, as it consists of nearly pure carbon, giving off a high degree
of heat without smoke. Its use, of course, necessitates the use of a
blower, but to this there can be no objection. Any coal which will burn
freely and clean, giving off no excessive smoke, and capable of almost
complete combustion, will give satisfaction in enameling; but it must
not be forgotten that the consumption of fuel is so large that both
price and quality must be carefully considered. Experimental tests must
be made from time to time. A cheap, common coal will never give good
results, and a good expensive coal will make the cost of manufacture
so great that the prices of the enameled articles will render them
unsalable. Any ordinary small factory will use from 2 to 4 tons per
day of coal, and it will thus be seen that the financial success of a
concern lies to a very great extent at the mouth of the furnace. Coke
is a good medium for obtaining the necessary heat required in enameling
if it can be got at a reasonable price. With a good draught a uniform
temperature can be easily kept up, and the use of this by-product is,
therefore, to be recommended. {301}