Exceptions to Slow and Low - Kiln Forming Myths 8

The principle of slow and low always applies.

Although the principle of attempting to get the effect you want at the lowest possible temperature with the slowest practical rate of advance should always be considered, there are times when it is not wholly applicable.

Among these are when working with small scale pieces, such as jewellery, and in general pieces below 100mm that are at least 50mm from the side of the kiln.  In these cases you can fire much faster, as the heat has less distance to travel through the glass to maintain an even heat.  You still should be using two stages – the first and slower to rise to the strain point and the second much faster one to reach the top temperature.  In these cases the target may have to be a little higher than in a larger, slower firing.

Another case is in fire polishing.  Fire polishing can often have a fast segment to avoid distorting the piece.  In this case you fire appropriately slowly for the thickness of the piece until you are past the upper strain point.  This can usually be taken as 540°C.  (For float and bottle glass the temperature is around 690°C).  As you have passed the brittle phase of glass by this time, you can advance the temperature quickly.  The objective is to achieve enough heat to change the surface, but avoid heating the interior to the softening point.  You may want to observe the finish of the surface, so that you can switch to the cool down phase of the firing as soon as the polish is achieved.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Stabilising Stringers

Stringers and rods never seem to stay where you put them.

• Glue them and they move after the glue has burned away.  
• Grinding a flat side to them seems a lot of work.  
• Easier, is to put them in the kiln and take them to a tack fuse to give a flat spot. But that takes a lot of kiln time.
• For stringers you can put a kink or curve in it by heating over a candle.  Rods require more heat than that. Of course, this is of no use for straight lines, and takes additional time.

A simple method which can be used with a tiny amount of glue, or not, is to add clear fine frit around the stringers and rods. This is enough to keep them from moving once the glue is gone due to the heat.

Assembled panel by Kathleen Watson with the stringers surrounded with fine clear frit which can be seen as white

The frit should be put on the assembled panel once it has been moved to and placed in the kiln.  Any movement will disturb the frit and defeat the purpose of keeping the stringer or rod in place.

The fired result

This was fused to a rounded tack fuse and no signs of the clear supporting frit is visible.

This is a quick simple method to stabilise rod, stringer and other small items that may shift in the firing.

Hand Finishing Jewellery Edges

Often jewellery scale pieces need to have their edges finished before the final wrapping or hanging on the necklace. This is frequently done by running the piece against the grinder, dumping it in water and then cleaning with a tooth brush or similar before the next process.  What is described here can be used on fused and “raw” glass both.

You can finish the edges of pieces by hand. 

Get a flat piece of glass – window glass is good for this.  You can put a fine grit such as 200 onto the glass and wet it to a paste. Place the edge of the jewel on the glass and begin rubbing with moderate pressure in an oval or figure of 8 motion.  You will be surprised at how quickly the edge is refined.  You can follow this up with finer grits.  Make sure you clean the jewel and the grinding plate thoroughly if you use the same glass plate for finer grits.

If you want a less messy - but slightly more expensive - method, use wet and dry sandpapers.  These can be found in grits from 200 to 6000, although you will not need to go beyond 1200 which will give you a smooth, shiny edge. These need to be kept damp too.  If you are planning to fire polish the pieces, you can stop at 400 or 600 grit.

This process avoids the water soak stage, can bring back into use the pieces you forgot to soak, and can be taken all the way to the finished edge.  If you are doing only a few pieces, it is much faster than a fire polish in the kiln.

Organic Burnout Marks

Occasionally there is a haze at the centre of the back of large pieces of fired glass. This seems to happen when a large piece of glass is placed over fibre paper (of whatever thickness) that has not been pre-fired. 

 This is based on my experience of doing large pieces on thinfire or other fibre paper with a relatively fast rate of advance. What seems to happen is that the edges of the glass soften enough and early enough that not all the binder in the fibre papers can burn out and the combustion gasses escape from under the glass. The resulting haze is the remnants of the combustion product fired to the surface of the glass.

I have found that flipping the piece over and taking the glass to a low temperature fire polish is enough to return the glass to its usual appearance. You can, for extra insurance, apply a devitrification spray, although I have not found it necessary.

You could, of course, work the back of the glass with pumice and cerium oxide to bring back the original shine without firing. But my impression is that the areas with haze are fractionally depressed into the back surface. This means that a lot of glass has to be removed to reach and polish the hazy areas.

Boiled Glass

This is a technique that will obtain a random, organic feel to glass that would otherwise be scrap (cullet) – remembering that you have to use compatible glass throughout. The principle is to take the temperature up high enough for the glass to begin to flow easily and bubbles to blow through and burst.

The results can be used as they come out, or they can be cut to provide points of interest in other work, or the glass pieces can be damed before firing to obtain thick pieces which can be cut into slices for other work. And I am sure, there are numerous other ways to use the resulting glass too.

The effects are rather like colourful molten rock with gases bubbling through. These bubbles mix the glass colours. So you need to be sure you do not use a wide variety of colours, or your result will be similar to the molten rock - muddy. Use a few contrasting colours, and ensure you include a significant proportion of white to maintain bright colours. Also remember that the hot colours – reds, yellows, oranges – opalise at high temperatures, so the transparents can be used as opals.

You can use whole sheets of glass or scraps. In either case, it is useful to start with a clear base to help avoid picking up kiln wash when the glass is moving about. The glass must be clean to reduce the incidence of devitrification. Stack you glass on top of the base glass in what ever order you like. Contrasting colours alternated give a strong result.

You can put shelf paper of 0.5 mm or thicker on the shelf or simply kiln wash the shelf with several layers of wash until the shelf surface is no longer visible through the wash. Use of thinfire is not recommended as the powder can be pulled into the glass.

If you do not dam the area to contain the glass calculate how far the glass will expand on the shelf, so that you do not put down too much glass and have it spill over the edge of the shelf.  

You can use bubble powder onto the base layer to promote the bubbling during the firing. However, if you are using cullet, you can just take the temperature up rapidly without a bubble squeeze, which will give you plenty of air pockets to burst through the layers of glass.

You can take the temperature up at about 300ºC per hour to 925ºC with no bubble squeeze and soak for 10 – 15 minutes. Then allow the kiln to drop the temperature as fast as possible to about 815 and soak there for around 30 minutes to allow the little bubbles to rise to the surface an burst too. Then reduce to the annealing temperature and soak for the thickness you calculated in preparation for the firing.

Precautions

You need to be careful in firing and annealing pieces using this glass. Any glass that has been fired to a high temperature tends to begin changing compatibility. So you need to be careful on your rates of advance, and on the annealing and cooling portions of the firing when using the glass in other projects. You may want to consider using a schedule for twice the thickness of the piece on subsequent firings.

There may be devitrification on the surface. You should sandblast or abrade away this devitrification in some way to be able to get a shining surface when you fire polish.

There may also be a number of pin hole sized bubbles at or just below the surface. These will close with a fire polish also.

Fire Polishing Jewellery

Sometimes jewellery pieces come out of the kiln with imperfections that need to be ground away.

You can do a quick shaping on the conventional grinder and then fire polish. However, you will not get a fire polish at a low enough temperature to avoid distorting the piece. The way to get a fire polished surface or edge is to make the edge less rough before putting the glass into the kiln.

After you shape the pieces on the grinder, make sure you scrub them well to remove any particles created by the grinding process. Dry them and then cover the rough areas with a white paint marker. This will indicate when you have removed the scratches caused by the previous rougher grinding.

Get out your wet and dry sandpapers. Start with the most coarse (about 200) and work the ground area until the paint has been removed. Keep the sandpaper and the area being worked damp. When all the white paint has been sanded away, wash, dry, paint and go to the the next finer grit. Repeat this with progressively finer grits – normally use the grit number twice the previous grit (larger numbers indicate finer grits).

Normally, going down to 400 or 600 grit will be enough to enable a fire polish at a low enough temperature to avoid distortion of your piece. This heat range will be at a low temperature tack fuse.

This smoothing process does not take very long and is much cheaper than buying several finer grinding bits - most grinder bits are 60 to 80 grit, although it is possible to get bits up to 600 grit.

Thick Uneven Pieces

Occasionally fused pieces come out of the kiln with one side thicker than the other. There are several things that need to be done for the present piece and for the future.


Level

First check how level your kiln is. The best for this is to begin with a check of the bed of the kiln. Check the level in four directions – left-right, front-back and the diagonals. If it is practical, wedge up the legs of the kiln to make the bed of the kiln as level as practical.

Then check how level your shelf is. Put in your shelf supports and then place the shelf on them. Again check with a spirit level the four directions. Place pieces of fibre paper under or on top of the supports to level the shelf. It is only after these checks have been made that you can consider firing your piece to help it return to an even thickness.  As part of your kiln maintenance you should check the level of your shelf at least monthly, if not every time you prepare to fire.

Variation in Thickness
Now that you know the shelf is level, you need to consider what the variation in thickness across the piece may be. The firing schedule needs to be more conservative than just for the thickest part. As the thinner parts will heat through more quickly than the thickest parts, you need to fire less quickly than you normally would for the thickest area. A rule of thumb – not always correct of course – is to add the difference of the thick and thin areas to the thicker and fire for that calculated thicknesses. This will make the firing schedule slower and so allow the thicker part to be the same temperature as the thinner. For example, a piece 6 mm at one side and 10 mm the other would have a difference of 4 mm. Add this 4 mm to the thicker 10 mm and then fire for 14 mm.

Temperature and Soak
You also need to consider the top temperature to use and the length of soak required. Glass flows relatively slowly at kiln forming temperatures. The conservative approach – one that allows further work if necessary – is to use the previous fusing temperature and extend the soak by at least twice the length of time on the previous firing, even perhaps to a couple of hours.

Bubbles
One thing that will happen is that the bubbles that previously were near the surface will rise and burst giving pin holes on this extended soak. So you should consider cleaning the bottom and putting the top face down on a separator between the shelf and the glass.   This will reverse the direction of flow for the bubbles. Few if any will break through the new top and there should be no pin holes when flipped.

Further Firings
When the piece is cool, check it for the even-ness of the piece all around. If it is not even enough, you will need to consider re-firing again. If you decide to do so, you should go no faster than the rate of advance as previously – probably even slower - but consider raising the temperature or extending the soak. Remember that achieving the heat work required at the lowest temperature is the guide line for kiln forming. So an extended soak should be preferred over a higher temperature, unless there are strong indications that a higher temperature is required.

Fire Polishing
Of course, you will now need to throughly clean the face down side and re-fire to fire polish the original top. The rate of advance should be the same or slower than the firing to even the thickness. Once you have achieved about 600C, a soak of about 30 minutes will ensure that the glass is thoroughly heated through. Then you can advance at a quick rate to the fire polish temperature with a soak of no more than a minute. This allows the surface to change without giving the rest of the glass time to begin to move.  Of course, a thorough annealing is required.

This procedure for re-firing  can be used when re-firing pieces for any reason. You only need eliminate the considerations on the uneven thicknesses.

Negative assembly

To get a crisp design finish to a fused piece especially with thin elements like stringers it is often recommended to fire upside-down. This means that the bubbles move toward the final bottom of the piece rather than rising and disturbing the design on the top.

Assemble the piece on your workbench as usual on top, but use a toothpick or small brush to dab some GlasTac or other glue near the two ends of each piece. Let it dry overnight and then carefully flip the whole arrangement upside down onto the kiln shelf.

If you don't like the glue and flip over technique you can try another. Draw your design onto Thinfire shelf paper in negative with a graphite pencil. If you have a strong light source you can draw on the reverse (printed) side and trace the negative onto the upper face of the Thinfire. Then assemble your pieces upside-down on the shelf. You can assemble the whole in the kiln and there is no need for glue.

When fired, clean the piece thoroughly and decide whether you want that texture or a smoother surface. If you want the smoother surface, put in the kiln for a fire polish. This is often known as “flip and fire.”

Fixing a Bubble

It is often recommended that large bubbles can be fixed by drilling out the hole if the raised part is thick, or by breaking the thin skin of the bubble and filling with a piece of glass or frit.

My experience leads me to say that "drilling and filling" is hardly ever successful. There always is a mark where the fill has been made, often there is a colour variation too.

There is another possible solution for small bubbles which are at the surface but not yet burst.

Turn the whole piece over and take to just below full fuse. This will allow the bubble to begin moving to the "bottom" of the final piece. Then flip over, clean really well and take to a fire polish - quickly up from bubble squeeze temperature and with no soak - to give a smooth surface, but not allowing it to get so hot the bubble comes back to the top. This will require observation to stop the temperature rise as soon as the surface is shiny.

Small burst bubbles

Small burst bubbles can be fixed by placing small “beads” of glass made from frit in the kiln. The appropriate sized bead can be placed in the hole and others arranged in a pleasant arrangement and all taken to a tack fuse. The beads are made by cutting small squares of glass 5-6 mm and taking them to full fuse. The glass will round up at the fusing temperature and give a number of completely rounded spheres with a flat bottom.

Prevention

Of course, the prevention of bubbles by inserting a bubble squeeze is the best solution.

Vertical Kiln Formed Holes

For vertical holes in frit-cast reliefs you can fill a drinking straw with plaster, cut while still wet and build the frit or cullet around it.

Already fused pieces can have the holes made much neater and smoother by using the above method in pre drilled holes. Another method is to wrap a thin strip of fibre paper around a pencil or end of a paint brush. Then push this circle of fibre paper into the hole. If this is the same height or a little less than the glass, it provides a clean fire polished hole, if the glass is taken to the high end of fire polishing temperatures.