Achieving a Matte Finish by Cold Working

Although sandblasting and then firing a piece can achieve a matte finish, there are several other ways to improve the quality of the final finish.

One of these involves the use of manual sanding after sandblasting in order to smooth out uneven spots and achieve a better final finish.

• Start with a 400 mesh diamond hand pad. It shouldn't be necessary to start out with a lower mesh (coarser) pad.

• Alternatively use wet/dry silicon carbide sandpaper. A combination of 400 mesh paper, followed by 600 mesh paper will work well.

• If you're using sandpaper, place a sponge between the paper and your hand for improved comfort and to improve the evenness of the final finish.

• An alternative to hand sanding is to use a electric sander or grinder, but be careful with the pressure you use, as it is possible to grind into the surface with a rapidly spinning surface. You also need to keep the surface wet to avoid heat build-ups.



You can also use a lathe with appropriately shaped wheels to give decorative effects to the object.

Firing for a Matte Finish

Glass can be fired to take on a satin appearance that is both appealing to the eye and pleasing to touch.

The first step toward the matte finish is to sandblast the piece after fusing, then fire to a temperature between 600C and 675C. A short soak - or no soak at all - is all that is needed.

The exact temperature needed depends on a number of factors, including:

• The specific glass being used. A soft glass such as black generally needs to be fired to a lower temperature than glasses that do not absorb the heat so easily. Every colour and type of glass will behave a bit differently, so experimentation and record keeping is critical.

• The grit and type of sandblasting medium. Generally, a grit from 120 to 200 is preferred, with aluminium oxide performing a bit better than silicon carbide – which can often lead toward some devitrification.

• The particular kiln being used. Your kiln is a bit different from any other one. Start with a temperature in the middle of the 600-650C range and adjust depending on the results you achieve.

• The finish you want will vary with only a few degrees difference. This means that you have to observe the firing. Make sure you keep good records of the specific firing schedule used so that you can make adjustments if needed for future firings.



Some variations can provide distinctive elements to the finished piece.

• Masking certain elements before sandblasting can provide contrasts of texture within the piece.

• Firing at a lower temperature for longer can give the results you want, without any additional marking on the bottom of the piece.

• To keep the matte texture, any subsequent slumping of the piece should be done at as low a temperature as possible.

Preventing Chipping When Using a Tile Saw to Cut Glass

One of the most common problems in using a tile saw to cut glass is the tendency for the saw to chip the edge of the glass as it completes the cut. This occurs when the blade of the saw has less glass to cut through. Excessive and uneven pressure and the lack of support cause this break-out.

It's possible to improve the quality of the cut by slowing down and pushing the glass through the blade more gently, but this seldom solves the problem completely. Pushing equally on both sides of the cut is also important to minimise the break-out.

One solution that does work is to provide support for the end of the bar. This adopts a woodworking method for preventing splintering at the ends of cuts.
Use a scrap length of pattern bar or other thick glass. Place it against the glass being cut. As the blade emerges from the glass being cut, hold the two pieces firmly together and continue cutting. The blade should immediately engage the second piece of glass. Once the saw blade entirely clears the first piece, you can turn off the saw and remove a chip-free slice from the pattern bar.

You'll need to trim off the ends of the scrap piece from time to time, but you can use the scrap over and over until it becomes too small to do the job.

This works best with a tile saw where the blade is below the cutting surface. When you use an overhead saw, the breakout is much rarer.

Dams for Pattern Bars

Once you have cut and arranged the glass for your pattern bar, you need to dam the bars in the kiln to prevent the glass spreading.

The materials required for forming the sides of the dam can be made from anything that is rigid and can withstand the heat of the kiln, e.g., cut up kiln shelves, rigidised fibre board, vermiculite board. The material being used to dam must be over 13mm and preferably around 25mm thick. It should be capable of standing vertically on its edge without support. Cut the dam material into strips at least as long as the pattern bars you're damming, and at least as wide as the bars are tall.

You also need fibre paper for lining the edges of the dam and keeping the glass from sticking to the dam. Cut the strips of fibre paper to line the walls of the dam and keep the glass from sticking to the dam material when you fire the kiln. Three millimetre fibre paper works best.

The width of the fibre paper should be 3mm narrower than the pattern bars are high. By cutting the strips shorter than the pattern bars you allow the bars to round perfectly on top and help prevent needling.

The fibre paper should go around all sides without gaps. They should have straight edges so the glass of the pattern bar does not leak between or underneath the fibre paper. The use of iridised glass on bottom and sides will provide a smooth release from the fibre paper and is a second option. It is also possible to line the fibre paper with thinfire paper to provide a smooth release, although it is more time consuming than using the iridised side of glass against the fibre. But do not combine thinfire and iridised glass. There is a reaction that leaves holes and craters in the glass.

Pattern Bar Box

Making a box for a pattern bar design that involves frit or lots of small pieces is necessary and simple.

Let's assume you want to make a pattern bar that's 25mm by 25mm by 200mm long. Start by cutting three strips of glass, each 25mm wide and 200mm long. Also cut two 25mm squares of glass. You can use any colour, but remember that the colour you choose will make up the outside of your pattern bar.

Assemble the three strips and two squares to make a small box, with one piece on the bottom and the others attached to form sides and ends. To do the attaching, use a hot melt glue gun.

The advantage of using hot melt glue to make a pattern bar box is that after the box is assembled, it can be filled with frit and other scrap outside the kiln, then easily carried into the kiln and dammed as usual. The glue will burn off during the firing. You can finish the box with a final strip of glass laid on top, but this isn't essential.

You then dam the box as for any other pattern bar.

Designing a Pattern Bar

Assuming that you are not going to just dump your scrap glass in a random pattern to form a pattern bar, you need to spend some time designing it.

The simplest kind of bar is composed of strips of glass which are stacked or assembled in the kiln, but there are many other more elaborate configurations.

Because of the additional annealing time required for larger and thicker items, most pattern bars range from 1" by 1" to no larger than 2" by 2". The length of the pattern bar can be any length, up to the maximum that will fit in your kiln.

The design process begins by thinking about the cross section of the bar. This is what will appear when cut and assembled. As a simple exercise, assume you are making a diamond pattern in the bar. You can draw this out using 3mm as the thickness (or 1.5mm if you are using thin glass). Rough out the pattern and then begin using 3mm as the grid. Remember that you will need to cut your strips 4mm or wider to obtain a clean break. As you plan it out you will see that you need one length at the base one half of the space remaining after you have laid down the first, central piece for the diamond. The next layer will have two strips for the diamond, giving a requirement for one strip to fill the space between the two for the diamond shape and two strips each one half the remaining space. This process goes on until the area is filled.

Pattern Bars

A pattern bar is a thick bundle of glass that has been fused together. These can be in the shape of a rectangle, or can be a thick pot melt – whether a disc or a rectangle. The length of the individual bars can be as long as your kiln allows, but needs to be practical to handle when cutting.


The basic steps involved in making a pattern bar include deciding on a design –whether controlled or random, cutting glass for the bar, assembling the cut glass into the desired bar shape, then firing to a full fuse. Once fired, pattern bars can be cut into slices with a saw - tile, glass, lapidary, or stone – which uses water for cooling and lubrication. The individual slices are then assembled and re-fused to make bowls, platters, and similar shapes. They can also be used as accents in any number of applications.

There is a caution about using pattern bar pieces. As the glass in the bars has been fired to a relatively high temperature, some of the characteristics may have changed. So you need to do a compatibility test before doing the main piece.

Designing Pattern Bars
Boxes for Pattern Bars
Dams for Pattern Bars

Float Glass in the Kiln

An important characteristic of float glass is that a very small amount of the tin is embedded into the glass on the side it touched. The tin side is easier to make into a mirror and is softer and easier to scratch than the air side. The characteristic of float glass having a molecular level of tin left on the “tin side” but not the “air side” is important to distinguish. There are short wave UV light sources to help determine this. The tin side gives a whiter glow than the air side. If any forming of the glass is planed after fusing, the tin side needs to be on the side being stretched, as when in compression the tin side will show a “tin bloom” similar to devitrification.

If the tin side is down on both sheets, and it is slumped into a mould there will be no tin bloom because the tin layer is stretched. If the tin side is up on both sheets and it is slumped into a mould there will be tin bloom because the tin layer is compressed. If you have placed the tin sides together, or on both the top and bottom, one of the tin surfaces will be in compression and so will show tin bloom. This is often mistaken for devitrification, and no amount of any devitrification solution will help.

A borax solution can help with the devitrification on float glass in some circumstances. It is not a perfect solution. This is because tin bloom and devitrification are often not distinguished correctly. But a high level of cleanliness and polishing the glass until squeaky clean is the best start.

The heat characteristics of Float glass depend in large part on which company manufactures the glass being used, so the temperature characteristics are given in ranges.

The softening point is around 760C

The annealing point is around 560—540C

The strain point is around 515-495C. The strain point being the temperature below which no further annealing occurs, although the glass can still be thermally shocked below this range.

Due to the robustness of float glass, it can be fired with a quicker initial temperature rise than glasses formulated for kiln forming. The down side is that it devitrifies very easily and very badly. Rarely can you perform more than two firings before the devitrification begins to become troublesome.

All window glass now seems to be referred to as float glass. However, the float glass process was invented in the 1950’s. Prior to that time, window glass was drawn. Float glass can use more iron in its composition, because it does not have to be drawn up out of a molten vat of glass as the drawn glass did and still does. Float glass is formulated to be stiffer at forming temperatures, whereas the drawn glass has to be flexible due to the mechanical stresses it is put under during the drawing. Except for low iron glass, the float glass has a distinct blue green colour when viewed through the edge. Drawn glass has a variation in thickness and is much paler when viewed through the edge. These visual differences can help distinguish the two kinds of glass, but are not foolproof.

More information on the general characteristics of float glass can be found here.

Stainless Steel Moulds

Stainless steel is sometimes called the almost the perfect mould material. It is lightweight, difficult to deform, and durable for a very many firings. Simple bowl forms are relatively inexpensive to buy — you can even use cheap stainless steel bowls. All you need to do is drill three or four small 1.5mm holes in the bottom for air to escape.
It often is a good idea to fire the mould to working temperature (say 650C) before attempting to kiln wash the form. This burns off the protective oils from the steel. Alternatively, sandblast the mould to clean it and give a small tooth for the kiln wash.

Stainless steel moulds do need to be covered with kiln wash. This is difficult to do when the mould is at room temperature, but it can easily be accomplished by heating the mould to around 150C, then brushing or spraying on the kiln wash while the mould is hot. The water in the wash will evaporate rapidly, leaving the protective elements behind. If you heat the mould too high the water will boil off, leaving gaps.

Also, it’s important to realize that steel contracts more than the glass. This is the opposite of ceramic, which contracts less than the glass. As a result, slumping on the outside of a steep stainless steel form generally works better than slumping on the inside.

Still, you can get away with slumping inside gentle bowl forms; just make certain it’s well covered in kiln wash. A sprinkle of a little kiln wash powder inside can also be considered. Be aware that slumping inside deeper forms may not work.

Draping over steel

Steel absorbs heat much faster than glass, so the glass suspended on the steel is cooler than the suspended perimeter during the heating and cooling cycles of the firing. This does not apply to slumping when the glass is supported on the edges, as so little of the glass is touching the mould at the start.

The fact that the steel “bleeds” the supported glass of heat while the unsupported parts heat up, requires slow heating with or without periodic soaks on the way up to ensure the glass and steel are the same temperature up to about 540C or the upper strain point of the glass.

I tend to be very cautious, and for 6mm pieces heat up approximately like the following:

100C/hr to 100C, soak 20

150C/hr to 200C, soak 20

200/hr to process temperature



When cooling, the steel is in closer contact with the glass, no special considerations are needed, so the normal annealing soak and cool are used.