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Fire polishing is the simple technique of returning
glass items to the kiln to melt them just enough to give a smooth, polished
appearance. It's typically used to round the edges of the glass between fusing
and slumping firings and takes place at a temperature around 1300 degrees F.
Fire polishing already slumped items is trickier. This is because the
polishing temperature is so close to the slumping temperature. Success is
sometimes achieved by firing just to where softening begins (around 1100-1200
degrees F) and watching carefully to make certain that slumping does not occur
and distort the appearance of the piece.
Because of the danger of changing the shape of slumped items, fire polishing
generally works best for flat items, rather than slumped ones. It also has the
limitation that the part of the item that touches the kiln shelf will not
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There are two main types of polishing sprays: those that need to be fired in
a kiln and those that do not. Each works quite differently.
Sprays that need to be fired are generally made of clear glass enamel that
matures at temperatures well below the normal slumping range. Typically, these
sprays are applied to the surface of the glass with a brush or sprayer, and then
the item is fired to around 1100 degrees F and allowed to soak for 15 to 30
These low-firing clear enamels (also called glass
fluxes) tend to be high in lead content, so they should not be used for surfaces
that will come into contact with food. Also, they are also only useful for
soda-lime glasses such as Bullseye and Uroboros. In addition, some people claim
that these glass fluxes impart a slight haziness to the glass. The two main
brands available are Back Magic from Fusion Headquarters and Flux 92 by Standard
Ceramics Supply, Inc.
A second type of polishing spray, which does not require heating in a kiln,
is spray oil. These oils, which make the surface of the glass appear wet,
will permanently change the look of the glass. They are applied by spraying an
even coat on the glass. Fingerprints and smudges will be highlighted, so make
certain that the glass is as clean as possible. After spraying, wipe off any
excess oil, paying special attention to areas of puddling. Allow at least half a
day for the oil to dry thoroughly.
Any clear oil, even salad oil, can be used in this fashion. One commercial
spray oil that works well is Varathane Natural Oil Finish #66 Clear. Test on a
piece of scrap clear glass to make certain that the oil dries clear and does not
Another kind of spray that works well but does not require firing is clear
enamel or acrylic spray. These sprays, which are widely available in hardware or
DIY stores, also impart a permanent, wet sheen. As with oil sprays, you should
test for yellowing on a scrap of glass, then spray evenly on a clean, dry
surface. Multiple coats may be used if necessary.
In addition to these sprays, items may be waxed to give a polished
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Acid polishing, which uses potentially harmful chemicals such as sulfuric and
hydrofluoric acid, should not be attempted by the novice. It requires extensive
attention to proper safety procedures. Not only are acid burns a possibility,
toxic fumes must also be dealt with and controlled. In addition, the acid bath
works best when heating slightly, adding to the potential risk.
In the typical acid polishing process, the glass to be polished is dipped
into an acid bath, then cleansed in water. In some cases a series of immersions
is used rather than a single dipping. Only a very short immersion in the acid is
necessary to produce a matte finish; longer periods of time in the acid bath
will yield a more polished appearance.
Despite the effectiveness of acid polishing, its potential dangers outweigh
the benefits for most warm glass artists. It should only be undertake by those
with the appropriate safety equipment and experience.
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TRADITIONAL POLISHING PROCESS
The traditional polishing process requires a steady stream of water and an
abrasive compound. The purpose of the water is to keep the glass from
overheating and cracking, while the compound helps to abrade the glass and
achieve a polished surface.
Generally, achieving a highly polished finish involves using a series of
finer and finer abrasives to wear away the glass by scratching the surface until
the scratches get so small they can't be seen. These abrasives are available
in many different sizes (called "grits"), ranging from around mesh
size 60 (a very rough grit used for initial grinding) to around mesh size 600
(an extremely fine grit).
In the traditional process, silicon carbide is mixed with water to form a
"slurry" of grit that is applied to a very slowly turning wheel. The
glass piece to be polished is held to the wheel and ground away using a
succession of finer and finer abrasives. Each change in abrasives (say, from 140
grit to 200 grit) requires a complete cleaning of the wheel or a separate wheel
Diamond grinding and polishing represents an improvement on this traditional
process. This technique uses removable discs that have been electroplated with
small diamond chips. The discs, which are also available in many different
grits, are affixed to the wheel (generally with magnets) and used with lots of
water for grinding and polishing.
Since the discs are removable and cleanup can be achieved with a simple spray
of water to wash the glass particles away, a single wheel can be used. Also,
diamond discs allow the wheel to rotate more quickly, allowing for grinding and
polishing to take place three or four times faster than with a silicon carbide
But using diamond discs and silicon carbide slurry alone won't result in a
highly polished surface. Instead, you must also use an additional surface to
complete the task. Cork surfaces, which are used with pumice and water, are
traditionally used for this final step, but most contemporary artists use a felt
surface and cerium oxide to achieve the final finish. An extremely highly
polished "optical" finish may require several applications of finer
and finer grits of cerium oxide.
Fortunately, warm glass artists rarely need to go through all the laborious
grinding and polishing steps described above. That's because our polishing
needs are often confined to small areas of the glass. Moreover, many of our
pieces emerge from the kiln relatively smooth, so that it is not usually
necessary to use the coarser grits.
It's often helpful to think of the finishing process as having four main
phases: rough grinding, final grinding, initial polishing, and final polishing.
Not all of these are always necessary, but it's good to have a basic
understanding of the materials and actions required for each of these major
-- Rough grinding
Begin with a 100 or 140 grit to remove large rough areas of glass. Grits as
coarse as 40 may be used; these will quickly remove a considerable amount of
glass, but they will leave a very rough finish that will require further
processing. As with all grinding and polishing activities, it's critical to
keep the glass item well lubricated to keep it from overheating and cracking.
For many warm glass pieces, the piece is already relatively smooth and this
step can be skipped. For small or easily accessible areas of glass (such
as edges), a conventional glass grinder can be used.
Grinder heads, like the discs described above, are also available in different
-- Final grinding
This step involves the use of a medium grit (270 to 300) to impart a smooth
finish to the glass. While the finish will be smooth when this step is complete,
it will not be clear or shiny; instead, it will appear whitish and dull.
If you begin with the step and your surface was still rough and really
required you to follow the "Rough Grinding" step instead, you will
still make progress. However, grinding will be a bit slower and the 270 to 300
grit surfaces will wear out faster than if the rough grinding step had been
-- Initial polishing
The initial smoothing process uses either a "smoothing" diamond pad
or a very fine abrasive grit (around 400 to 500). For many items, such as
jewelry or small castings, that only require the polishing of a limited surface
area, the grinding phases are not necessary and the polishing process can begin
with the initial polishing operation.
-- Final polishing
This is where you switch from diamond (or silicon carbide) abrasives to
optical polishing compounds like pumice and cerium oxide. Pumice is generally
used with cork surfaces, while cerium oxide is used with felt. (In most cases,
the cork and the felt are synthetic, rather than natural.)
To final polish, mix a small amount of your abrasive (pumice or cerium oxide)
with water to form slurry about the thickness of paste. If you're using a
wheel, thoroughly wet it and then apply a bit of this mixture to the surface.
Hold the glass against the wheel to polish.
If you're using a Dremel or other movable tool, secure the glass firmly,
wet it down, and apply the paste directly to the glass. Then move the tool to
the glass to polish.
Regardless of the tool used, it's critical that you not allow the glass to
become dry. Add more water and wet slurry frequently. A finished surface will
shine with no visible scratches.
Sometimes it is necessary to repeat this process with a finer grit of cerium
oxide, but most pieces can be polished sufficiently well with just a single
Note also that if you are using a felt surface to polish you will need to
"charge" the surface prior to using it the first time. To do this, you
simply apply a thick paste of cerium oxide and water to the pad, work it into
the surface, and allow it to dry overnight. Felt pads work better after they
have been used a few times, so don't become frustrated if your first few
attempts fail to achieve perfect results.
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MANUAL APPROACHES TO POLISHING
Manual approaches to polishing can be successful, but of course they tend to
be more time consuming. If you have enough time and patience, you can mix
abrasive grit with water and rub it on the glass surface by hand.
More commonly, abrasive sandpaper (3M's tri-mite, for example) is used.
"Sandpaper" is really a misnomer, since these sheets are really coated
with silicon carbide or aluminum oxide. They are used wet and applied to the
surface of the glass, starting with rougher grits and finishing with the
smoother. For hand polishing most kiln-worked items, using a 400 grit paper is
generally sufficient, so long as it is followed by a final polish with felt and
In addition to abrasive papers, both 3M and Abrasive Technology make flexible
diamond hand pads that can be used for manual polishing. These pads, also
available in grits ranging from around 70 to 600, can be used wet to grind and
polish glass surfaces. Manufacturers of the pads include 3M, Abrasive
Technology, and Crystalite.
With all of these approaches, the key is to start with coarser abrasives and
move to finer ones. It's also essential to keep the glass wet while grinding
and polishing, and to thoroughly rinse the glass between steps. A final hand
polish with felt and cerium oxide will help you achieve a brilliant polish.
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The variety of machines that can be used for polishing is simply astounding.
Polishing equipment ranges from automatic vibrating reciprolaps, which polish
flat surfaces automatically by vibrating underneath the piece, to simple Dremel
tools. Lathes, belt-machines, power tools, and grinders have all been adapted
The advent of easily removable pads and belts, which can be switched from
grit to grit as necessary, makes it possible to grind and polish glass in many
ways. The method of using the pads depends on the particular machine and pad
used, but all have in common that they are used wet on the surface of the glass.