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Enamel Preparation
by Woodrow W. Carpenter
Volume 1, Number 2, March 1982
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The traditional method of enamel preparation was described by
Cunynghame1 and
Chapin2.
Large chunks of enamel were wrapped in a piece of cloth and broken
with a hammer. The cloth was to prevent the particles from
flying about, getting into one's eye or being lost. When the
enamel had been broken into pieces about the size of a pea, they
were washed to remove any lint. Quoting Cunynghame
verbatim: "The enamel is placed in a very hard mortar,
about 8 inches in diameter, preferably of Scottish or Villon
granite, with a pestle of the same material. A little clean
water is poured on to it, to prevent the chips from flying, and
then it is pounded into small pieces with the aid of the
mallet. The mortar may be laid on a bag of sand to prevent
its being broken by the shock. Afterwards the enamel is
ground up with the pestle to the size of ordinary sea
sand." Two paragraphs later: "After the
enamel begins to become as small as sand, a milky substance seems
to be disengaged and to fill the water, which lies above the
enamel. This consists of some of the colouring matter of
very fine particles of enamel and of potash and soda. If any
of it is left in, the enamel when fired will be opaque and
dull. Hence it must be washed away by agitating the pounded
enamel in water poured into the mortar and then pouring off the
fluid. This must be done until the enamel remaining is in
fine even grains, looking like perfectly clear, clean, fine
sand. The size of the grains may be such as will go through
a fine sieve with meshes 75 to the linear inch." Five
paragraphs later: "Opaque enamels need not be washed,
except to remove any little dirt that may have got in, and, as
will presently be seen, some coatings of enamel cannot be washed,
but must be put on in a state of impalpable powder. So thin,
however, are the layers thus used that they are fused up into
transparent enamel." As far as we can determine, he did
not expand on this statement presently or later. If he had,
he probably would have said that low firing enamels made at this
time were practically water soluble.
Unfortunately, the art-enameling community has always been
isolated from the glass and enamel scientists. As a result,
Cunynghame cannot be criticized, because he did not have the
benefit of knowledge gained during the eighty years following
publication of his book.
During the last decade of the nineteenth century, glass scientists
became very involved with the durability of glass. The first
published work of note was by Foerster3
in 1893. Much has been published up to the present. A
few of the major works are listed as references.
Let us start with the enamel as it is removed from the pot and
poured onto a metal plate to cool. At this stage, it is a
round flat disk perhaps eight inches in diameter and one half inch
thick. Depending on the composition, some are quite durable
as to water, acid and alkali. Others are not so
durable. In any case, they all are at their maximum
durability at this moment. Surface tension caused the glass
to assume a minimum volume, thus tightening the network structure
at the surface, forming what we might call a fire polished
surface. Any free alkali at or near the surface is
vaporized, resulting in a skin which is a little more durable than
the interior. When
cool, the enamel cakes are ground or crushed. The normal
European method is ball milling, while the normal method in the
United States is crushing with hardened steel rolls. The
principle advantage of the latter method is fewer fines are
produced. Ground enamel
is slightly less durable than the cake or lump form. When
two immiscible phases, such as a gas and a solid are brought into
contact, the solid will adsorb a thin film of the gas.
Adsorption is to be distinguished from absorption, which involves
the bulk penetration of the structure of a solid by a gas and is
governed by laws of diffusion.
Air is a gas which normally contains some water. Thus, at
certain humidity and temperature conditions, all solid surfaces
will adsorb a thin film of water. A fire polished surface
will adsorb only a thin layer, known as physical adsorption.
Such layers are weakly bonded and can be removed by a slight
increase in the temperature of the solid.
When glass is broken, an atomically clean surface is
exposed. Immediately, this clean surface reacts with the air
by a process known as chemisorption. If the certain humidity
and temperature conditions mentioned above exist, the surface will
adsorb a thin film of water. Unlike physical adsorption,
chemisorption consists of strong bonds and the water cannot be
removed by a slight increase in the temperature of the glass.
The thin film of water reacts with the glass. First is an
ion exchange of alkali and hydrogen ions (explained below), and
second, the formation of sodium hydroxide and/or sodium
carbonate. If the humidity and temperature conditions
change, the sodium hydroxide and/or sodium carbonate may
crystallize and cease to react. If conditions change so the
crystals can absorb water, reaction will restart.
The extent of the reaction depends upon the composition of the
enamel, and the precautions exercised by the manufacturer,
distributor, and enameler.
The scene now switches to the enameler's studio. According
to most books, all enamels are immediately washed and stored wet,
in small jars or bottles.
At least three different steps are involved in the reaction of
water with ground enamel. The first, is ion exchange of
hydronium (H3O+)
or hydrogen ions from the water with alkali ions in the
glass. Second, is the partial hydration of the
silicon-oxygen network of the glass. Third, is the
dissolution of the glass into the contacting solution. 
Figure 1 is a schematic diagram showing water in contact with one
surface of glass, assuming the other surfaces are protected.
The dry glass at the bottom, contains alkali ions at the original
concentration. As one proceeds upward to the glass surface,
there is a decrease in the concentration of alkali ions (dotted S
curve) as a result of their replacement with hydronium ions.
In this layer of partial exchange, the network structure of the
glass is intact, and the only change is replacement of one ion for
another. Closer to the surface, the network can become
partially hydrated by reaction of silicon-oxygen bonds with water. 
This partial hydration leads to a more open structure than in the
original glass; ions from solution and water molecules can
penetrate through this partially hydrated or gel layer with
mobilities much higher than in the glass network that has not been
broken up by reaction. (1)
The exchange of alkali ions in the glass and hydronium ions from
water can be described with the equation: Na+
(glass) + H3O+
(solution) = Na+ (solution) + H3O+
(glass). (2) As
reaction (2) proceeds the solution becomes more basic, and the
rate of dissolution of the silicon-oxygen network becomes more
rapid. At extended time
of reaction, the amount of alkali appearing in solution becomes
proportional of time. Furthermore, silicon and other glass
constituents are found in the solution. These results
suggest that the glass dissolves into the solution by reactions of
the type: 2 H2O
+ SiO2 = H4SiO4
(3)
H2O + CaO = Ca(OH)2
(4)
3 H2O + A12O3
= 2A1(OH)3 (5)
H2O + Na2O
= 2 Na(OH) (6)
H2O + PbO = Pb(OH)2
(7) In reaction (2),
the sodium cation (ion with a positive charge) was used as an
illustration because it has the greatest mobility in a glass
network. Actually, all cations react with the hydrogen ion
as shown in reaction (2), but at different rates.
The rate of the above reactions depend greatly on the composition
of the enamel and to some extent on the amount of water used, as
well as the temperatures of storage, and whether or not the jar is
tightly sealed. When a sufficient amount of the enamel has
been dissolved, the mass will 'set up' like concrete.
Enamels have been made with durability so low that they would 'set
up' in a matter of days. Enamels can also be made with
durability so high that it takes years.
Since the attack is proportional to time, it is obvious that fine
particles would be completely changed to a gel before large
particles. The composition of the gel will vary some,
depending on the composition of the enamel. In any event, it
is composed of metal hydrates which will not form a glass at a
normal firing temperature. Therefore, it is desirable to
remove the fine particles which have a high degree of
deterioration. The custom has been by elutriation as
described by Cunynghame. This does not remove the gel or
deterioration from the larger grains, which may give off water up
to 1000°F or higher, leaving metal salts which will not be taken
into solution by the glass at normal firing temperatures,
resulting in white specks. If some combination of acids
and/or alkali could be used to completely dissolve the gel, there
remains that portion of the glass where there is a partial
exchange of hydrogen ions for alkali ions. Once most of the
hydrogen ions and water molecules have been driven out with heat,
there remains a silica rich area which is more refractory and of a
much lower expansion than the bulk of the glass. This part
of the glass will have a dull appearance and some opacity due to
some water being retained. Perhaps these silica rich areas
can be removed with hydrofluoric acid, but how do you stop just
short of dissolving too much silica and end up with surfaces too
rich in alkali? There
is an alternate method to elutriation. It is called screen
separation. Three screens, 100 mesh, 200 mesh, and 325 mesh
should be sufficient for most purposes. Enamel classified
through 100 mesh and remaining on 200 mesh is ideal for good
transparents. That which passed through the 200 and remained
on 325 can be ground in a mortar and pestle to pass through 325
and used for painting. Normally, opaque enamels can be used
without screening out the fines. In rare cases, it might be
helpful to remove particles finer than 325 mesh. 
Figure 2 shows a set of two screens along with a collecting pan
and a cover. Note a couple sizeable lumps of frit in the
screen will aid in keeping the wire cleared to speed up the
process.
Figure 3 shows how the screens nest making the operation
easier. Of course, the enamel can be screened in single
screens without nesting.
No doubt some enamelers will feel some extremely fine powder will
adhere to the grain surfaces throughout the screening and wish to
remove it by washing. Alcohol is ideal for washing
enamel. It has high affinity for water and evaporates
readily. Ethanol (ethyl or grain) should be used, even
though it is more expensive. Methanol (Methyl or wood) is
poisonous if taken internally or with prolonged breathing of the
fumes. Although we have
pointed out water can be a source of problems with some enamels,
others are quite resistant to water and little or no problem will
develop. Our concern is the teaching of washing as a
fundamental principle. It should be taught as a special
operation for a special purpose, if taught at all.
If we were determined to use certain enamels and were concerned
with obtaining transparency, we would purchase it in lump form,
and grind it in a mortar with a transparent plastic cover with a
hole in the center to allow the handle of the pestle to stick
through. We would grind a short time, screen, regrind,
screen, continuing until enough enamel of the proper mesh was
obtained. We would wash, only if necessary, with water or
alcohol. Any left over enamel that had been exposed to
water, should be discarded. Any ground enamel, which has not
been exposed to water, should be stored in a desiccator.
Again, it is stressed, all enamels do not require this degree of
pampering. Enamel being
attacked by water is not a unique phenomenon. Water attacks
all glass, especially when freshly broken or ground into a
powder. An enlightening experiment is to place a piece of
window glass in distilled water and ad a few drops of phenolphthalein.
No reaction will be indicated. Grind the piece of glass into
a powder, add water and a few drops of phenolphthalein. The
solution will immediately turn pink, indicating the presence of
alkali in the solution.
Of historical interest, are the following two excerpts: The
first from Cellini4,
written 1568: "We have a proverb in the craft which
says, 'Smalto sottile e niello grosse.' 'Enamel should be
fine, niello should be coarse', and that's just what it is.
You put your enamel in a little round mortar of well-hardened
steel, and about the size of your palm and then you pound it up
with very clean water and with a little steel pestle especially
made for this purpose of the necessary size. Some, to be
sure, have pounded their enamels on porphyry or serpentine stone,
which are very hard and more over have done this dry, but I now
think that the steel mortar is much better, because you can pound
it so much cleaner."
The second excerpt is from the third edition (published in 1906)
of Cunynghame, page 91:
"But, since this edition was published, Mr. Charles Tomes,
F.R.S., has made some interesting experiments which shed new
light upon the subject." They will be found in the
August number, 1900, of the Journal of the Society of Arts.
His conclusion is, "that the apparent mud only consists of
finer particles of the very same composition as the coarser stuff,
and that these fine particles, especially on the surface, become
very quickly agglutinated by the heat of the furnace, entangling
between and beneath them an infinity of small bubbles but that,
when coarser particles are fired, they run together more slowly,
and then the air escapes for the most part, the little which
remains forming large bubbles, which do not practically interfere
with the transparency."
"The experiments of Mr. Tomes undoubtedly bear out the
general proposition put forward by him. He concludes that
grinding enamels in paraffin oil* is not better than to grind them
in water." *(In the U.S., paraffin oil is called
kerosene.) "In
this as a practical result, I am unable to agree with him, for
although enamels kept under water suffer but little change,
enamels kept for many days in a state of fine, damp, mud
undoubtedly appear to undergo decomposition."
The observations of both Tomes and Cunynghame were correct.
If Tomes applied the fine enamel immediately after grinding, the
small bubbles would have been the only difference. And, had
he applied the enamel thin, he might have eliminated most of the
small bubbles. Cunynghame was correct in that small
particles deteriorate faster than large particles. Small
particles have more surface area in proportion to their volume
than do larger particles.
When working with transparent enamels, one of Cunynghame's remarks
quoted earlier in this article should be considered a fundamental
principle: "So thin, however, are the layers thus used,
that they are fused up into transparent enamel."
We hope those who experiment using alcohol for washing will share
their experience with Glass on Metal Magazine. 
References
-
Cunynghame, Henry, On
the Theory and Practice of Art-Enamelling Upon Metal.
Archibald Constable & Company, Westminster.
1899. Pp. 48-51
-
Chapin, Howard. How
To Enamel. John Wiley & Sons, New
York. 1911. Pp. 1-7.
-
Foerster, F.Z. Instrumentenk.
Vol. 13, pg. 457. 1895.
-
Ashbee, C.R. The
Treatises of Benvenuto Cellini on Goldsmithing and Sculpture.
Dover Publications, Inc., New York. 1967. Pg. 17.
Other References
Pye, L.D. &
others. Introduction to Glass Science, Plenum Press,
New York. 1972. Pp. 513-529.
Tomozana, M. & others. Treatise
on Materials Science and Technology, Vol. 17. Academic
Press, New York. 1979. Pp. 41-69.
Doremus, R.H. Glass Science.
John Wiley & Sons, New York. 1973. Pp. 213-228.
Scholes, S.R. Modern Glass
Practice. Industrial Publications Inc., Chicago.
1951. Pp. 262-266
Shand, E.B. Glass Engineering
Handbook. McGraw-Hill Book Company, Inc., New
York. 1958. Pp. 91-102.
Wang, F. & Tooley, F.V. "Detection of Reaction Products Between Water and
Soda-Lime-Silica Glass", Journal American Cer. Soc., Vol.
41. 1958. Pp. 467-469, 521-524.
Simpson, H.E. "Measuring
Surface Durability of Glass", Ceramic Bulletin, Vol.
30. 1951. Pp. 41-45.
Eppler, R.A. & others. "Resistance of Porcelain Enamels to Attack by Aqueous
Media," Ceramics Bulletin, Vol. 56. 1977. Pp.
1064-1070.
Ceramics Bulletin Vol. 60.
1981. Pp. 618-622.
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