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GLASS POLISHING

Just click on the glass polishing technique that interests you.

 

FIRE POLISHING

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 polish.

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POLISHING SPRAYS

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 minutes.

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 yellow.

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 appearance.

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ACID POLISHING

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 set-up.

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 slurry.

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 steps.

 

-- 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 grits.

 

-- 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 performed first.

 

-- 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 application.

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 cerium oxide. 

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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MACHINE-AIDED POLISHING

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 for polishing.

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.

 
 
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