Longevity of Borax as a devitrification agent

It is true that Borax is water soluable. However, the borax has done its job by preventing the devitrification, so it does not matter whether it has or has not disolved, nor whether it is inside or outside.

Borax as a flux for paint in excessive quantities has the effect of corrosion on the paint or enamel it is mixed with. It is not actual corrosion, just that its effects are like that. The borax expands when wet. The expansion is very little, but over time "pops" off the paint - the time scale is 50-80 years. This happens on the inside of windows where the paint is. So it is not an inside/outside issue, just one of moisture.

But this irrelevant in kiln forming applications when attempting to prevent devitrification, or even to correct existing devitrification. The subsequent possible disappearance of the borax will not matter to the appearance of the piece. It has been reported that borax covered sushi dishes going through dishwasher cycles in a restaurant for years show no devitrification after the presumed disappearance of the borax. In fact, the proprietary devitrification solutions that contain lead would not be applicable in this food containing situation.

Other references to devitrification are:
Homemade devitrification solution
Description of devitrification
Temperature range

Devitrification Prone Glasses

"Are there specific glasses that are more prone to devitrification, and knowing that, what steps can you take to try to avoid it?"


Glasses that are formulated and tested compatible for kiln forming are less likely to devitrify than other art glasses.

Opalescent glasses even if tested compatible for kiln forming are more likely to devitrify than their compatible transparent counterparts.

Yes, you can fuse some of the transparent glass made by a single manufacturer - Spectrum transparent and especially the water glasses are most often compatible within certain limits. But you will find that the edges show devitrification almost always. When using glass untested for compatibility, capping with clear glass often helps in reducing or preventing devitrification, as the clears seem less prone to devitrification than coloured glasses

You can clean very well and hope for the best, or you can clean and then use a devitrification agent - normally a flux or low firing glass in suspension - and spray or brush it on. If it is one of the low firing glasses in suspension, make sure you put it on before taking it to the kiln, as it will stick to other things when fired.

Another method is to avoid staying in the devitrification range of temperatures very long - both during temperature rise and cooling.

A description of what devitrification is


The temperature range in which devitrification occurs


A homemade devitrification solution

Annealing

Stress is induced into glass during cooling through the outsides of the glass cooling more quickly than the interior. This contraction causes residual stress. Annealing is the process to relieve that stress. The annealing soak temperature is determined by a number of factors, of which coefficient of expansion, viscosity, exposed surface, and thickness are some. “The relief from stress happens because of a process of viscous flow. At the annealing point it can take place within a few minutes whilst at the lower annealing temperature…. It can take a few hours.” (Dictionary of Glass, Charles Bray, p.27)

The above statement is applicable to glass of a single colour from one manufacturer. When combining colours in kiln forming, the colours absorb and give off heat at different rates and so you need to allow more time for the annealing – relieving of heat induced stress – to occur. 

The annealing soak has the purpose of allowing all the glass to be the same temperature (within 5°C) from top to bottom, and side to side. The annealing occurs during the slow cool past the lower strain point.  The manufacturers give annealing and strain points for their glass. These should be observed, rather than anything pre-programmed into your kiln’s controller.

Note that the stress of incompatible glass cannot be relieved by annealing.

Also, each time the glass is taken to a temperature above the annealing point, it must be annealed again.  There is no short cut to this.

There are more notes on annealing here.

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