Overview of Metal Preparation
by Woodrow W. Carpenter
Volume 1, Number 1, January 1982
Metal preparation, at
least for this article, is defined as the treatment a metal
surface receives just prior to the application of enamel.
Metal preparation can vary from practically none to quite elaborate
procedures. A review of literature reveals a wide variety of
methods, all of which apparently worked satisfactorily with the
particular set of conditions described. Methods of
preparation can vary because of many factors including, but not
limited to materials used, desired final effect, or scale of
During the spinning or
stamping of copper shapes, light oil is applied, making the
operation easier and extending the life of the tooling.
Normally, this oil is removed prior to shipping. If some
remains on a relatively simple, flat or shallow shape, spread the
oil as evenly as possible with you finger and dust some dry 80
mesh enamel over the oil. Pop it into the furnace and watch
it flame, smoke and perhaps a little soot fly around. When
the enamel is glossy, take it out. Results may not be
fantastic, but in most cases, will be quite respectable.
Transparent enamels may have a few bubbles, but many times opaque
enamels look as good as when elaborate metal preparation is used.
If the copper shape
came without oil, everything is not lost - some can be
applied. Pine oil, lavender oil, fuel oil, 3&1 oil,
turpentine, Vaseline, glycerin, baby oil - they all work
surprisingly well. Spread them on with your finger, or use a
brush. Sometimes better results are obtained if the dusted
piece is inserted into the furnace and removed after the oil
flashes. When smoking ceases, return piece to furnace.
Repeat two or three times then complete the firing.
Admittedly, this untraditional approach will never become the most
popular technique - there are too many shortcomings.
However, for classes of children, brushing some lavender or pine
oil onto the copper shape then sifting on enamel may be quite
Once water is
introduced into the system, things change, because water and oil
do not mix. Also, to obtain the quality most enamelers
rightfully desire, refinements are necessary. Metal
preparation may, but not necessarily, consist of four steps:
Removal of oil and grease.
Removal of tarnish and scale or oxide.
REMOVAL OF OIL AND
GREASE. Usually called cleaning. There are three
are seldom used other than to dissolve any asphalt type resistant
that may be present. Many of the solvents are a fire hazard
and also five off toxic fumes. The thin film usually left on
the surface of the metal is very detrimental. Water will not
wet this film, thus when enamel containing moisture is applied, it
will crawl leaving bare areas.
Burning off seems the
ideal method for the art enameler. Temperature of the
furnace is not critical, because the flash point of oils and
greases is well below enameling temperatures. Length of time
also is not critical. However, if you plan to remove the
fire scale later from copper, it is best not to prolong this
time. For convenience, a normal enameling temperature at
1450-1500°F can be used. When copper is heated, it
turns dark within a few seconds, then almost immediately, if much
oil is present, a small flash appears and an iridescence moves
across the copper, indicating all organic matter has
vaporized. Remove the copper and allow to cool. A very
thin film of black oxide will be on the surface.
In some situations,
metal preparation can end at this point. If loose scale is
present, it can be brushed off with a clean paper towel.
Opaque and even transparent enamels can be successfully applied
with most techniques. Again, most enamelers want more
control of background pattern when working with transparents,
necessitating further metal preparation.
If the burn off is
extended for a longer time, a heavy layer of tightly adhering
oxide forms. The outer portion being black copper oxide (CuO)
and the inner portion, at the metal-oxide interface, being red
oxide (Cu2O). As the layer
of black oxide builds up, it prevents sufficient oxygen reaching
the interface to form the higher oxide.
Silver and gold do not
combine directly with oxygen. Therefore, no oxide is formed
during burn off. In fact, if an oxide was present, it
decomposes when gold reaches 400°F, or at 570°F
Aluminum forms a
colorless protective film of oxide which remains colorless even up
to its melting point. Since aluminum melts at about 1220°F,
burn off temperature should be no higher than 1100°F.
Heating at 1000-1100°F for five minutes is definitely all
that is required to prepare 3002 aluminum for enameling.
A simple way to
determine if burn off has been sufficient, is to flood the metal
surface with water. If water drains off in a continuous
layer, no grease or oil is present. If the film breaks,
beads up, or pulls to one side, the surface is greasy and burn off
should be repeated.
is a washing operation to remove oil and dirt from the surface of
the metal. Chemical cleaners consist of alkali and soap, or
detergents along with wetting and buffering agents. The
alkali combines with soluble oils to form soaps, a process called
sponification. Insoluble oils are emulsified by the
soap. To be effective, these cleaning solutions must be
operated at a boiling temperature. When only light oils are
involved, a hot solution of trisodium phosphate is usually
sufficient although it is used quite extensively by industrial
enamelers. Chemical cleaning is not very practical for the
It is possible to
scour the metal surface with household cleaners, such as
Barkeepers Friend, then rinse with water to see of the surface
retains a continuous film of water. Cleaners containing
chlorine must not be used on aluminum. It causes the enamel
to chip off edges. Care must be taken that the rag or
scouring pad used does not become contaminated with oil, merely
spreading it from one area to another. A simple and sure way
is to burn off all oil.
REMOVAL OF TARNISH
AND SCALE. Copper tarnish from normal room atmosphere is
basic copper carbonate. It is soluble in acids and
decomposes when heated to 400°F. Silver tarnish is
normally silver sulfide. It is soluble in nitric and
concentrated sulfuric. It decomposes when heated to about
1515°F. In general, there are two methods used to
remove tarnish and scale:
Pickling is an
operation in which acids remove tarnish and oxide or scale from a
metal surface. Acid is not a cleaner, it does not
react with oil and grease to form soluble products. Oils
merely float to the top of an acid solution, to be picked up on
the metal surface when removed. All grease and oil must be
removed from the metal surface before putting it into the
Acids most often used
are nitric, sulfuric, and hydrochloric. Each acid has its
own peculiar properties and they are not always
interchangeable. Metals react differently to acids.
Those above hydrogen in the electromotive force table are
dissolved by all three acids forming metal salts and giving off
hydrogen. Metals below hydrogen will not dissolve in acids
unless an oxidizing agent is present. Copper, silver,
platinum, and gold all fall below hydrogen.
concentrated sulfuric are oxidizing acids. They first
oxidize copper or silver then react with the oxide. Dilute
sulfuric and hydrochloric must have an oxidizing agent present to
react with copper or silver. Atmospheric oxygen can function
as this oxidizing agent, but etching takes place very
slowly. Hydrogen peroxide or ammonia persulphate are
suitable oxidizing agents.
It is generally known
that gold and platinum dissolve only in aqua regia, one part
concentrated nitric with three parts concentrated hydrochloric.
and concentrated sulfuric are normally used at room temperature,
because they give off noxious fumes when heated. Dilute
sulfuric can be heated to about 160°F before fumes become
acid is a powerful dehydrating agent, not only absorbing free
water, but it also withdraws the elements of water from organic
matter. When concentrated sulfuric acid is diluted with
water, a great amount of heat is liberated. Great care must,
therefore, be exercised in diluting all acid. Water should
be placed in a Pyrex container and acid slowly added, stirring
occasionally. WATER MUST NOT BE ADDED TO ALL ACID.
This results in the sudden absorption of a large amount of heat by
a small quantity of water, turning it into steam and producing a
spattering of hot liquid. Several years ago, the author of
an enameling book stated the rule, "Never pour water into an
acid," and most all books on enameling since, has repeated
the rule. This applies only to concentrated sulfuric.
Nitric acid purchased
from a laboratory supply house contains 69.0 - 71.0% nitric
acid. When chemists speak of dilute nitric, they usually
mean the standard given in the "Handbook of Chemistry and
Physics" published by CRC Press, Cleveland, Ohio. This
is 195ml of 69% nitric acid and 805ml of water, or approximately
one part to four parts by volume. This provides a strength
of about 17.6% acid by weight. Some enamelers use a weaker
solution, others use a stronger solution up to one part of each,
which provides a strength of 40% water. Consequently, when
developing new enamels at Ceramic Coating Company, we always use
this strength to determine acid resistance.
from a laboratory supply house contains 36.5 - 38.0% acid.
Standard dilute is 258ml of acid and 742 of water. This
provides a strength of about 10.5% by weight. Industrial
enamelers frequently use a little stronger solution, up to one
part of each providing a strength of 20% acid by weight, normally
used at room temperature.
from a laboratory supply house contains 95% acid. Standard
dilute is 168ml of acid and 832ml of water. This provides a
strength of about 25.7% acid by weight, which can be used at room
temperature. Commercial enamelers frequently use a strength
of about 16% by weight, or one volume of acid to nine volumes of
water, at room temperature. Others use a hot solution,
approximately 160°F with a strength of 8% by weight, or
one part of acid to 20 parts water, by volume.
Immersion Time - 5 minutes
Cold Solutions - 70°F
Hot Solutions - 160°F
N.T. - Not Tested
Figure 1 shows the
solubility of several materials in all three acids. This
table can be helpful in selecting pickle procedures. For
ordinary trays, plaques, etc., dilute nitric is suitable; it
readily dissolves black copper oxide, slowly dissolves red copper
oxide and copper to produce a fresh bright surface. In
situations where a heavy layer of red oxide must be removed, it
would be more efficient to first dissolve the black oxide in
dilute nitric, then into dilute hydrochloric to remove the red
oxide. Finally, a quick dip in dilute nitric to remove a
little copper, thus exposing a bright new surface.
Champleve and die
stamped ware with delicate designs pickled in sizeable lots might
lose some detail if pickled very long in nitric. Thus,
commercial enamelers normally use dilute hydrochloric or hot
sulfuric which dissolves the oxides, followed by a quick dip in
Copper can be removed
from the surface of sterling silver by first heating to oxidize
the copper, then pickle in hydrochloric or hot dilute sulfuric to
dissolve the copper oxide without attack to the silver (depletion
gilding). To obtain the surface purity desired, this
procedure may have to be repeated 8 to 10 times.
Bright dip is a
mixture of one part concentrated sulfuric and one part
concentrated nitric with a trace of hydrochloric. Many
enamelers use this to improve the brilliancy of
transparents. Copper is first pickled in nitric, then given
a quick dip of a few seconds in bright dip. The copper
surface will be brighter, a little more yellow in color, and
appear to be smoother.
Figures 2, 3 and 4 show a copper surface magnified 150
times. Figure 2 was annealed at 1500°F for 10
minutes to enlarge the grain size. It was then pickled with
dilute nitric and rinsed with water. Some grains show up as
white, others show as gray, because they are tilted slightly and
reflect a different amount of light. The dark areas are
either grains tilted enough that the light is reflected outside
the lens or trenches eaten by the acid causing a shadow.
Nitric acid produces
more distinct grain boundaries than either sulfuric or
hydrochloric. However, we see no indication that this has
any effect on the brilliancy or color of transparent enamels.
Figure 3 shows the same piece of copper, as shown in figure 2,
after two seconds in a bright dip solution. The grains have
been evenly attacked and the grain boundaries are not very
evident. This is a much smoother surface than the one in
figure 2. It is surprising that bright dip initially attacks
copper very fast and then immediately slows down.
Apparently, the metal surface becomes passive.
Figure 4 shows a copper surface after five minutes in concentrated
nitric acid. The plate was hung by the top right hand
corner. Deep vertical grooves are quite evident, indicating
Many people fear the use of acids and utilize substitutes which
are less effective, but in many cases satisfactory.
Vinegar and Salt.
A damp cloth with vinegar, daub into common salt and scour the
metal surface. Vinegar contains 4 to 8% acetic acid, which
reacts with salt forming a weak hydrochloric acid. This
procedure is effective for small ware. Much of its
effectiveness is due to the mechanical action of scrubbing.
If considerable pickling is to be done in this manner, it is best
to wear a rubber glove to prevent finger stains.
Metal shapes may be
submerged in a solution of vinegar and salt. Use about 45
grams (1.6 oz.) of salt to one cup (8 fl. oz.) of vinegar.
For hydrochloric to be effective, strength should be about 12% and
this strength cannot be obtained using 4 to 8% acetic acid.
Figure 1 shows the solubility of copper oxides in this solution.
Sulfuric Acid Salts.
Sodium bisulfate, also known as sodium acid sulfate, or niter
cake, a by-product from the manufacture of hydrochloric and nitric
acids, is sometimes used. It is a white crystalline
powder. Dissolve approximately 10 ounces in a quart of
water. This can be used cold, but it is much more effective
if heated. Figure 1 shows the solubility of copper oxides in
Many things can be used to mechanically remove tarnish or oxide,
i.e. emery cloth, abrasive stones, wire brush, slurry of water and
scrap enamel, Bright Boy, and finally steel wool. Great care
must be taken when using steel wool to prevent small pieces from
getting into the enamel and producing black wiggly designs where
they are most unwelcome.
Lea compound, a
greaseless polishing agent, can be applied to a buffing wheel and
used for both cleaning and removal of tarnish and scale from
DRYING, should accomplish two things:
Neutralize any acid remaining on the metal
Retard discoloration of the metal surface.
Should acid be left on
the metal surface, the enamel will blister and become
discolored. If the metal surface dries slowly, it will
usually have a mottled pattern of discoloration.
For simple shapes, the
easiest way to neutralize the surface is to rinse it well with
water. The easiest way to prevent a mottled pattern of
discoloration is to wipe it dry with a clean cloth or paper
towel. Enamel can be sifted directly after rinsing, thus
eliminating the need to dry. If some time is to elapse prior
to coating all or any part of the surface, it is desirable to dry
the metal surface.
Many methods have been
suggested in literature. An early one was to apply saliva to
the surface and distribute it with a clean finger, followed by
rinsing again with water. This would neutralize a small
amount of acid. This method appears to be limited to small
pieces rather than for mass production.
Moistening a clean
cloth with soapless detergent and wiping the surface, followed by
rinsing in water is one recommendation. This should
neutralize acids. Any metal salts present should be picked
up by the cloth. Subsequent rinsing might be more thorough
as the detergent would reduce the surface tension of the water.
Some authors, starting
with Cunynghame, recommend a weak solution of ammonia, followed by
rinsing in water. Again, this should be effective as a
Scrubbing with pumice
or plaster of paris has been suggested. This would
mechanically remove any metal salts left on the surface. We
suggest using scrap enamel, since it is usually on hand and works
enamelers use a weak, hot solution of trisodium phosphate.
As well as neutralizing, the warm ware dries more rapidly,
reducing mottled discoloration.
Others use a hot soda
ash and borax solution. Three pounds of soda ash and one
pound of 10 mol. borax to one hundred gallons of water, usually at
a temperature of 150-160°F.
who process several pieces as a batch, might wish to take into
consideration that copper nitrate is more soluble in water than
copper chloride. Copper chloride is more soluble in water
than copper sulfate.
discoloration has no effect on opaque enamels. However, it
is usually evident after the first firing of transparents and
perhaps even after several firings. As mentioned earlier,
simple pieces can be dried with a cloth, but some batch or complex
pieces cannot be easily dried with this method.
Drying can be
accomplished by placing the piece into an oven at about 300°F.
Blowing with hot air has been suggested, but care must be taken
that the air contains no oil.
recommends placing the ware immediately after rinsing into clean
saw dust. Some specified boxwood sawdust, others specified
oak sawdust. This works quite well.
Some enamelers simply
leave the ware submerged in water until time to enamel. Then
remove and dry it.
Cunynghame and one of
our European competitors, recommend that the metal be placed into
the furnace for a short time until the metal begins to change
color prior to applying transparents. We doubt the logic of
burning off the grease, pickling in sulfuric, going into bright
dip, drying with a clean cloth and then putting it into the
furnace to obtain a thin coat of oxide prior to applying
In conclusion, there
are many ways to prepare metal for enameling. Simple methods
are not necessarily bad, and complicated ones are not
automatically good. Select the one that fulfills your needs
at the moment, even though it may differ from previous