Tacking Freeze and Fuse to Base Glass

The question has been asked:

I'm wanting to add some freeze fuse pieces on to float and just fire to a tack fuse … in one firing instead of two …[to avoid] losing the detail on the freeze fuse pieces. The top temperature on freeze and fuse is 720°C versus a … float tack temperature of 787°C. [can this be done?]

My response:

What you are doing with the freeze and fuse process is sintering the glass particles together by holding at a low temperature for a very long time.  This binds the glass together without altering the overall shape of the object. 

Sintering

There is no reason why you cannot sinter the freeze and fuse piece on top of a base glass, if you pay attention to one major thing.  The freeze and fuse object will shade the heat from the base glass.  If you do not slow the rate of advance enough, you will break the base glass by creating too great a temperature differential between the part under the freeze and fuse piece and the uncovered part.

Another element to be considered, is that the frozen object is damp.  This will need to be dried by a slow ramp or it will further complicate the uneven heating problem.

Scheduling the Rate of Ramp

Choosing the rate of increase in temperature is determined by the dimensions of what is being sintered.  One widely practiced method is to double the total height and fire for that dimension.  For example, if the freeze and fuse is 8mm high, add that to the 6mm base and fire for 28mm – (6+8=14)*2 =28mm.

Another slightly less cautious approach is to multiply the total height by 1.5 and use the firing conditions for that thickness.

Determining the rate of advance for the thickness you have calculated – by either method - can be aided by using the Bullseye chart for annealing thick glass.  Look at the final cooling rate in the chart for the nearest thickness. In this case, use the one for 25mm.  The cooling rate is given as 90°C per hour.  If the glass can safely cool at that rate, it should also survive that speed of heating at the start.

If you chose the 1.5 factor, the thickness to schedule for will be 21mm.  This is between the 19mm and 25mm thicknesses given in the Bullseye chart.  The cooling rate given for 19mm is 150°C and and for 25 is 90C. As 21mm is almost the mid point between the two, you can halve the difference in rates (150 and 90) to give 120°C as the rate of advance. Although in both schedules using these rates of advance for the described circumstance, I would add a soak at 250°C for 20 minutes, to be cautious.

Remaining Parts of the Schedule

Sintering Soak

The length of soak for the sintering stage can be the same as the soak for the freeze and fuse, as you will be both sintering the glass pieces together and to the base glass too.

Anneal Cool

The annealing soak and cool should follow the rates given for the calculated thickness - in this case for 21mm or 28mm.

The Bullseye chart Annealing Thick Slabs can be used for all types of soda glass (which includes float glass) to determine the soak times and cooling rates.  You only need to make alterations for the annealing temperatures.  The annealing temperature I use for float glass is 540°C. 

The first two stages of cooling are 55C each, so simple subtraction from the annealing soak will give the temperatures for each stage of the cooling. If we use the calculated 21mm thickness, the soak time will be 3.5 hours at 540°C.  Then the Bullseye chart's displayed cooling rate of 20°C will apply from 540°C to 485°C, and the cooling rate of 36°C will apply from 485°C to 430°C. The final cooling rate of 120°C will be from 430°C to room temperature.  The chart for these adaptations is described in the post about adapting the Bullseye chart for annealing.  The reasons behind these operations are given in the ebook Low Temperature Kilnforming.

Hake brush

Bamboo handle hake brush

The hake (pronounced hah–kay) brush was developed in the far east.  It has several variations – the original consisted of a group of bamboo brushes bound together in a line.  These are still made and used. Many modern hake brushes have a broad wooden handle with a wide line of hairs.  These brushes are made of very fine, soft hairs - often goat hair is used. 

Flat wooden hake brushes

The flat hake brushes are most often cheaper and in a wider variety of sizes than the bamboo ones.  I prefer the bamboo for the feel in the hand that the broad handle gives.  With the longer hairs, it holds more moisture and delivers even amounts of kiln wash even with long strokes. 

Use

These brushes can hold a lot of moisture and deliver it evenly.  This makes it good for laying  down large areas of even colour in watercolours, and in glass painting. The same characteristic makes it very good for coating shelves with kiln wash.  The brush should be filled liberally with the paint or kiln wash. The brush should be gently shaken to remove any excess. Hold the brush nearly vertical and let the bristles barely touch the surface as you move along in smooth sweeps across the surface.  This allows the kiln wash to be evenly spread with very few brush marks.

Maintenance

One drawback of these brush is that the fine soft hairs are difficult to bind into the ferrule.  This results in the brushes often shedding hairs onto the shelf as it is being coated. A tip I learned from Bullseye is to treat the new hake brush with superglue at the base of the hairs. It does not have to be super glue.  It can be any runny glue, or diluted PVA.  I prefer super glue, even though it is reported to have some sensitivity to moisture. You can work the glue into the centre by using a needle to poke at the hairs to move the glue toward the centre of the bristles.  The glue binds the hairs in addition to the binding at the ferrule, and so keeps the brush from shedding. 

I did this on my bamboo handle hake brush a couple of years ago and it is not yet shedding hairs during applications of kiln wash.

Make sure you clean the bristles immediately after using to avoid any material drying among the hairs and causing them to break when next used.  To clean the brush, you only need running water run through the bristles.  Do not scrub the bristles against anything.  The hairs are delicate.  Set the brush aside horizontally to allow water to drip off and the hairs to dry.  Setting the brush upside down when wet allows water into the bindings of the hairs.  Putting it with the hairs down onto a surface deforms the hairs, making it difficult to straighten them later.

A hake brush is among the most useful tools to put kiln wash onto shelves and moulds because it holds so much moisture.  It does require maintenance to ensure the hairs do not shed and that the delicate hairs are not broken.

Large Tiles for Kiln Shelves

Pizza stone in use

People frequently wonder if other materials than mullite can be used for kiln shelves.  Mullite is used for its strength and very small expansion, even at high temperatures, as used in ceramics firing.  There are other materials that can be used in kilnforming of glass such as refractory fibre board, and ceramic pizza stones,  the best of which are made from mullite.  This post is about using ceramic floor tiles.

An unglazed floor tile, 11 x 11 inches

Structural Soundness

A major element in obtaining and using a floor tile is how sound it is.  Tapping the tile to determine whether the sound is a low toned ring or a dull thud is important.  There may be invisible cracks within the tile.  A dull thud is an indication that the whole tile has one or more cracks in it, or that it has not been fired high enough to completely vitrify the clay. A low frequency tone indicates there are no cracks and that it has been fired sufficiently high.

Flatness

The first thing you need to do is make sure the ceramic tile is flat and without undulations before using it. To test this, get a straight edge and move it along the tile to look for any slivers of light coming through underneath the straight edge. Any light or variation in the amount indicates depressions that can produce bubbles during the firings. Do this test at least twice at right angles to each other.  Take note of the depressed areas (or even possibly high areas) to know where these uneven areas are to work them out of the tile. 

You can do the above test in the showroom.  Another more accurate means of checking is more difficult to do in a sales area.  Place a line of dark powder, say black glass powder, and with a straight edge held vertical to the shelf, drag the powder across the shelf.  Where there are dark patches is an indication of depressions.  The area and depth can be seen from the spread of the visible powder and to some extent the density of the colour.

Making Shelf Flat

If you buy two of these large tiles, you can rub them together face to face in circular motions. The abrasion marks will show the high spots, with the low spots clear of those marks.  This will indicate the amount of work needed to get the whole surface even.  The smaller the unmarked areas, the less grinding will be required. You can add an abrasive with some water to form a slurry and continue to grind until everything is even. The use of water with the abrasives is important to eliminate dust which might be harmful, and to ease the grinding process.

The above is a manual process.  If you have a large enough flat lap, you can mechanise the flattening process.  Using decreasing grit sizes, you can grind the shelves level with a high degree of smoothness. You do not have to use a grit of less than 200, as the tile structure is even more coarse than that.

If you can't find unglazed floor tiles, you need to look at the back of the tiles.  Many floor tiles have a grid pattern on the back to ensure sufficient adhesive is used.  This makes getting the back, unglazed side flat more difficult or time consuming, because they will need to have the grid ground down to the lower surface.  In this case, it may be that you need to sandblast the glazed side before making sure it is flat.  The sandblasting can make a flat tile uneven by unequal times spend on various parts of the tile, so you have to check after sandblasting for the flatness.

Drying kiln wash

“Dry your kiln wash between coats and before firing.” 

This is a frequent statement when talking about renewing kiln wash on shelves and moulds.  The main reason given seems to be that there will be less risk of creating bubbles by evaporating moisture.  The air drying will reduce moisture in kiln is a second reason.

There are some difficulties with this statement and reasons.

Drying between coats of kiln wash means you are applying liquid over powder. This can promote clumping and streaking through a too rapid absorption of water by the dry kiln wash. Also, it makes applying kiln wash a lengthy process.  It is not like painting a door or even a floor, where you must allow drying to avoid streaks. 

Credit: Ceramicartsnetwork.org

Applying kiln wash by brushing is smoothest if all coats are done at once.  This is what happens if you spray kiln wash on your shelves and that gives a smooth surface.  If it were otherwise, drying between coats would apply to spraying too.  Drying between coats promotes streaks in the applied kiln wash that needs to be smoothed before use.  This of course, does need to be done after the kiln wash has dried.

Drying before using the shelf or mould is unnecessary. The evidence I have to offer is that I frequently fire within an hour of applying fresh kiln wash to a cleaned shelf. I have had no problems with creating bubbles or glass picking up the kiln wash. The shelf dries, with a moderate rate of advance, long before the glass settles into the texture of the surface.  It is only as the glass settles into the contours of the kiln wash that it seals air, or any other material, under the glass.

The pigment in most kiln washes is to tell you which shelves have not yet been used.  If they are fired dry at even moderate temperatures, the pigment disappears.  Then you have removed that indicator of freshly prepared shelves or moulds.

Drying of kiln wash before use in not necessary.  If you wish to be cautious, air drying will be enough to avoid any problems with moisture.

Sharp points on rectangles

At the conclusion of firing pieces with right angles or sharper shapes you often find very sharp needle points at the corners.

This is a result of the expansion of the glass as it heats up.  At top temperature, the glass piece is larger on the shelf than when you put it in cold.  The amount of this expansion is related to the thickness of the piece and the temperature it has been fired at.

As the glass cools, it contracts.  The contraction at corners and points has greater effects on the glass than at the sides.  The corners are contracting from two sides rather than only one.  This makes them a little more resistant to contract and often leaves a little of the glass stuck at the furthermost point of expansion as it contracts.

I have found the best prevention of sharp points on the corners of rectangular pieces, and those with even sharper angles, is to nip off the tiniest bit of the corners. This very slight blunting of the corners allows the glass to expand and then retract without the corner or point catching on the separator and so creating the sharp needles.

Further information is available in the e-book: Low Temperature Kilnforming.

Recovering from Devitrification

An explanation of what devitrification is, can be found in the link.

Mild devitrification is generally a smeary appearance on the surface.  Most often this can be corrected by either removing the surface, adding a flux or putting another surface over the piece.

mild devitrification
photo credit: Bullseye Glass Co.

Removing the devitrified surface

Sandblasting and grinding are two common methods of removing the surface. If you have access to a sandblaster, this is the easiest method of removing the surface.  You can remove the surface with manual methods too.  You can use wet and dry sandpapers, starting with coarse ones and proceed through grades to at least 400grit (0.037mm).  The flexibility of the sandpapers is that they can conform to uneven surfaces that tack fusing provides, to remove devitrification in depressions as well as the high bits. Diamond hand pads and sheets do the job more quickly, but are more expensive.

Acid etching is another surface removal method. There are various etching creams on the market which will remove the surface. You need to apply and leave for a long time to allow the acid to work on the glass surface.  It is best to keep the acid paste damp to enable the acid to work over a long period.  A piece of cling film will work well.

Making a new surface

You can provide a new surface by using devitrification sprays.  There are both commercial products and do it yourself ones that work.  The do it yourself product is a borax solution.  The method for making the solution is given here.

Borax powder

You also can give the devitrified surface a new one by covering it with clear powders.  Powders sifted evenly over the surface until there is a thin covering over all the piece will give a new surface concealing or covering the devitrification.  Fine frit does not work so well, as more needs to be sifted over the surface.  This will not be applicable to tack fused pieces, as the whole piece needs to be taken to a contour or full fuse to make sure the powder or frit is completely smooth.  This will make the tack fused areas flat.

Left to right - devitrified surface, powder covering, fired piece
Photo credit: Bullseye Glass Co.

When dealing with devitrification, the whole of the surface should be treated, not just isolated areas.  Treating isolated areas will most probably leave a difference in appearance between the treated and untreated areas.  It is not worth the risk of having to fire yet again.

Dealing with devitrification usually involves removing the devitrified surface or making a new one.

Making Thin Sheets

The question of how to make thin sheets arises from time to time.  Unless you are a glass manufacturer, it is unlikely you can make large, thin glass sheets.  But you can approximate making thin sheets by two methods that I know.

Sintering

One of these is sintering.  This is firing the glass to a low temperature and soaking for a long time.  The common form of this is powder wafers. 

By using a screen to deposit an even layer of glass powder you can make very thin, but delicate sheets of glass.  The procedure I would use is a screen of about 45 – 60 threads per inch.  This is coarse enough to allow the powder through, but not so fine as to “reject” large amounts of the coarser particles. 

You can screen the powder directly onto a kiln washed shelf, or onto Thinfire or Papyros.  You will not be able to move the unfired powder on a sheet of paper or fibre paper without changing the thickness and shape of the screened powder.  It must be laid down onto the separator directly on the shelf.  You can of course, move the shelf to the kiln if you can get in without tipping it.

Method

Support the screen about 3mm above the surface to allow the powder to fall through.

Make a ridge of powder at one end of the screen.  Using a smooth straight edge wide enough to cover the whole of the screen, lightly spread the powder from the starting end to the other. Then repeat drawing the powder to the starting end.  Make about five repeats of this – that is 10 passes, to get enough powder laid down to form about 0.5 to 1mm sheet.  You will need to experiment with the number of passes to get what you want.

Do not try to press the powder through the screen.  That will only wear the screen out quickly and may tear it.  Each pass should be a light spreading of the powder.  It is heavy enough to fall through the screen without additional force.

You could, of course, just sift the powder over the area you want to cover and judge by eye how even the layer is.  It is possible that your observation is good enough, but it is more likely that you will have thick and thin areas.  Often even at sintering temperatures, the thin is pulled toward the thicker, leaving small or large holes.   By screening the powder, you know you will have an even layer

Firing

The kind of schedule to use to sinter the glass particles together without changing their structure is the following:

220°C to 482°C , soak for 60 mins
55°C to 593°C, 10 minutes
28°C to 665°C for 5 mins
as fast as possible to 482°C for 30 mins
28°C to 427°C, no soak
55°C to 370°C, no soak
110°C to 50°C, no soak

This will work for most fusing glasses.

This slow firing allows enough heat to penetrate the glass grains that they will stick together without changing shape or developing holes.  I admit the anneal cool is very cautious.  You can experiment with quicker cools if you want to speed the process.

  

Pressing

This is a technique of thinning already existing sheets of glass.  Typically, you will have a 6mm or thicker piece of glass that you want to be 3mm or less.  Paul Tarlow has described this kiln pressed glass very well in his books and on the fusedglass.org site.

In essence, you use a pair of kiln shelves.  Kiln wash both shelves.  Place the glass to be thinned on one shelf.  At the outer edges of the shelf put down spacers of the thickness you want the glass to be after pressing.  This will keep the upper shelf from settling down too much and more importantly unevenly.  Place the other shelf, kiln washed side down, on top of the glass.  Be sure the spacers are in places where they can support the upper shelf.

If you are thinning from 6mm to 3mm, normally you do not need any additional weight on top of the upper shelf.  But the thinner you want the glass to be, the greater the weight needs to be.  It could be another shelf, fire bricks or steel weights.

When scheduling the annealing remember you must take account of the mass of the weight on top of the glass.  You will need a much longer temperature equalisation soak and a much slower annealing cool.  

Further information is available in the ebook Low Temperature Kiln Forming.

Pot Melt Contamination

Pot melting occurs at temperatures above that for which kiln washes are designed. This means the kiln wash most often sticks to the back of the melt.

If you put only fiber paper – Thinfire, Papyros, or standard 1mm or 2mm fibre paper – at the bottom, the dripping glass will tear and move it about.  It also tends to incorporate fibers from the refractory papers into the melt.  It is best to avoid fibre papers of any kind on the base.  Using fibre paper around the edges of dams, if you use them, is better than simple kiln washing of the dams.

From wikihow

If you have a sandblaster, it is easy to take the kiln wash off leaving a matt surface. You can live with this for many purposes, but if you want a more polished surface you can take the melt up to fire polishing temperature to shine up the surface. You will need to flip this over and fire again, if the original top surface is what you want to present.  Or if you like the new shiny surface, use it as is.

If you are going to cut the pot melt up for other uses, there is no need to fire polish as the surface does not matter, only the cleanliness, and removal of contaminants.


There is another thing you can do to avoid kiln wash contamination.

The best solution appears to be to put a disk or rectangle of glass on top of fibre paper. It can be clear or any colour you wish, but needs to fill the area enclosed by the dams. This seems to keep the fiber paper from tearing and being incorporated into the glass, even though the base will have the fibre paper marks.

It also works very well when you are confining the melt to get a thicker disk. Make sure you have kiln washed the sides of the container or dam very well, in addition to 3mm fibre paper arranged so that it is 3mm narrower than the expected final thickness, or any excess glass may stick to the dams. The means of arranging the fibre paper around the dams is given here. You may need to grind the marks off the edge of the disk, but this is much easier than grinding it off the bottom.

Schedules for Steep Drapes

I have been asked for a schedule for draping in the context of a tip on steep straight sided drapes.

What you are trying to do with a steep drape is two things. One is to compensate for the heat sink that the glass is supported by, and the second is to compensate for the relative lack of weight at the outer edge of the glass.


The supported glass transmits its heat to the support, leaving it colder than the unsupported glass. This often leads to breakage due to heat shock at much lower temperatures and slower rates of increase than glass supported at its edges. My experience has shown that - contrary to what I recommend for other kinds of firings - a slow rise with short soaks at intervals up to the working temperature works best. The reason for these slow rises and soaks is to try to get the support and the glass to be as nearly as possible at the same temperature throughout the rise in temperature. The soaks help ensure the mould is gaining heat without taking it from the glass.

The other problem with steep drapes is that the edges of the glass begin to drop more quickly than the area between the support and the edge. This leads to the development of an arc that touches the mould side near the bottom before the glass between the edge and the and the support. Extended soak times are required to allow the glass to stretch out and flatten. If this is done at high temperatures, the glass will thin - possibly to the extent of separating.

So the requirements for a firing schedule on this kind of drape are slow increases in temperature with soaks to avoid thermal shock, and an extended soak at the (low) forming temperature.

Whether using steel or ceramic moulds, I use a slow rise in temperature to 100C with a soak of 15 minutes. I then increase the rate of rise by 50% for the next 100C and give a 15 minute soak there. For the next 200C I raise the temperature at twice the original temperature rise, again with a 15 minute soak. The glass and mould should now be at 400C. This is still at the point where the glass could be heat shocked, so I only increase to 2.5 times the original rise rate but use this rate all the way to forming temperature.

Each kiln has its own characteristics, so giving schedules is problematic. 

•  A side fired kiln will need slower heat rises than a top fired one. 
• The closer the glass is to the elements, the slower the rate of increase needs to be. 
• The kind of energy input - electric or gas - has an effect. 
• The thickness of the glass is also a factor in considering what rate to use. 
•  The size of the glass in relation to the size of the support is important - the greater the differential, the slower the heat rise should be. 

So in making a suggestion on heat rises, it is only a starting point to think about what you are doing and why you are doing in this way.

I have usually done this kind of draping in top fired electric kilns where the elements are about 250mm above the shelf, and about 120mm apart. In the case of a 6mm thick piece about three times the size of the support area, I use 50C/hr as my starting point. This is one third of my usual rate of temperature rise. However you must watch to see what is happening, so that you can make adjustments. You should observe at each of the soaks, so you know how the glass is behaving. It will also help you to pinpoint the temperature range or rate of advance that may be leading to any breakages.

On steep slumps, the temptation is to use a high temperature to complete the drape. This is a mistake as the glass will be more heavily marked and tends toward excessive stretching and thinning. What you really need is a slow rate of advance to a relatively low temperature. If you normally slump at about 677C, then you want to do this steep, straight sided drape at 630C or less. It will need a long soak - maybe up to an hour. It will also need frequent observation to determine how the drape is progressing. So plan the time to make yourself available during this forming soak.

Annealing is done as normal, since the mould and glass are more closely together and will cool at the same rate.

The original tip on the set up of a steep straight sided slump is here.

Damming Ovals

There are various ways of damming oval shapes in kiln forming. Some of these are outlined here.

One set of methods depends on having a soft surface such as ceramic Fibre board or vermiculite.

Photo from Clearwater Studio

You can wrap your shape with fibre paper. For this you need to cut a strip or strips 3mm narrower than the height of the piece you are wrapping. You then stick sewing pins down through the fibre paper and into the shelf of fibre board or vermiculite. This will be easiest if you use 1 to 3mm thick fibre paper, as the pins must not contact the glass – the pins will stick to the glass if they do.

You can cut a form out of ceramic fibre board and use that as a dam. You can pin this to the base fibre board or allow it to merely rest on the board. It is possible to cut arcs from fibre board and place them around in sections. In this case they will need to be pinned together so they do not move apart. Staples can form the attachments. You can make your own – larger – ones from copper wire.

You can buy stainless steel banding which needs to be lined with any separator – batt wash or fibre paper.

Bonny Doon stainless steel dams

You also can layer fibre paper up to the height required – remember 3mm less than the thickness of the piece. You then need to fasten the layers together to avoid movement between the layers.


If you are firing on ceramic kiln shelves the same materials can be used but need to be supported a little differently.

If you are wrapping the piece on mullite shelves, use some pieces of kiln furniture to block the strips up against the glass. The thicker the glass, the more weight will be pushing out against the dams and the sturdier the dams will need to be. Make sure the strips contact the shelf evenly- if you have gaps, you'll have leaks.

The disadvantage to this method is that the glass can take up the irregularities of the kiln furniture.

You can use fibre board with a void cut out to the shape required and place it on the shelf.

You can also use layers of fiber paper around the shape and pin the layers to each other. This is the same method as used on ceramic fibre board.

Again stainless steel can be used to form the dam. Remember to line the steel with fibre paper that is 3mm narrower than the height of the piece.



In all these cases of dammed forms, the edges will be of varying degrees of roughness and some cold working will be required.

Slumping unknown glasses

I had a recent request for help from an old friend who has taken up kiln formed glass. The problem is common enough, that (with her permission) I am adding it to the tips section.

I tried an experiment today to use some of my nice (non-fusing) glass. I cut at 270 mm diameter circle from a 3mm thick sheet and wanted to slump it into my 270 mm bowl mold. I set the mold up carefully and checked it was dead level in all directions and that the glass was absolutely centered on it. I have no idea what the COE is so decided just to use the S96 recommended slumping temperature of 650C. When I checked the kiln no more than 2 minutes after it had reached 665C, the glass had slumped almost to the bottom of the mold but it had slumped very asymmetrically. There was also a small burp on one side which has never been an issue when slumping bowls in this mold before.

The schedule I used was as follows:
200C/hour to 540C, 0 hold
650C/hour to 665C, 10 hold
Then standard S96 anneal programme

Also, the edges were still a bit rough from the cutting, i.e., they hadn‘t fire polished at all. Can you help?

Finding out about the softening characteristics of the glass

Slumping a single layer of glass with unknown characteristics – the CoE is not really relevant – requires that you watch it and other similar ones until you have established a slump temperature for the glass.

There is a way to do it:
Cut a piece of glass 305mm long by 20mm wide. Support it 25mm above the kiln shelf with the posts being 290mm apart. Put kiln furniture on top of the glass where it is supported. Make sure you can see the shelf just under the middle of the suspended glass when you are setting up this test. You can put a piece of wire or other dark element there on the floor of the kiln to help you see when the glass touches down.

Set the kiln to fire at 100C/hour to about 680C. Peek at the suspended glass every 5-10 minutes after 560C to see when the glass begins to move. Then watch more frequently. If your kiln has an alert mode on it, you could set it to ring at each 5C increase in temperature, otherwise use an alarm that has a snooze function to make sure you keep looking. When the glass touches down to the witness sitting on the shelf, record the temperature. This will approximate the slumping temperature in a simple ball curve mould.


Getting smooth edges

You need to have smooth edges before slumping. You can fire polish the piece of glass to get rounded edges, or you can cold work the edges with diamond hand pads, working from the roughest to the finest you have available. If that does not give you the edge you want, you will need to fire polish before you try to slump.

You can do at least two things to find the fire polish temperature. You can do a little experiment by using the cut off pieces of the glass and roughing them up a little before putting in the kiln. Make sure you can see it through the peep hole(s). Set the kiln to fire at about 250C to say 750C. Look in from about 700C to determine when the edges begin to round.

The other is to put a strip of the same glass in with the slump test and set the kiln to go up to 750C rather than just 680C. You can check on progress just as for the separate firing to determine the fire polish temperature. I think about 40C above slump temperature should be enough, but your test will determine that.


Avoiding uneven slumps

Most uneven slumps occur because of too fast a rate of increase in temperature. The piece can hang up on the mould sometimes causing the glass elsewhere to slide down to compensate. The real difficulty in the schedule was the 650C/hour rate up to the top temperature. This was so fast that the glass at the edges would have the opportunity to soften and so hang before the centre was soft enough to begin to bend. 150C / hour would be fast enough from 540C to achieve the slump.  In fact, 150C/hour all the way to the slumping temperature would be fast enough.  The glass reacts well to a steady input of heat rather than rapid rises, even with soaks at intervals on the way up.

Other things can be done too. You mentioned the edges were rough from the cutting. This can cause difficulties of hanging. To avoid that, you should smooth the edges before placing the glass on the mould. A further precaution against uneven slumping is to give a slight bevel to the bottom edge so that it can slip more easily along the mould.

You had already done the leveling of the mould and the centralization of the glass on the mould. These are two other things that can cause uneven slumps.


Avoiding “burps”

The glass slipping a long way down the mould is often accompanied with burps or bubble like up-wellings. These are both indications of too high a temperature being used to slump. I would begin looking at the glass from about 600C in the slumping of any unknown piece of glass. That would apply to any new configuration of the glass or mould too. The fact that the glass slid to the bottom and had a burp means that the temperature was too high and too fast. Once you have established the lowest slumping temperature, by watching to see when it begins, you then can add about 30 minutes soak to that temperature. The length of this soak will have to be determined by observation and experience, though.

A slow heating allows the glass to be at an even temperature throughout its thickness. A rapid rise with a thick piece will sometimes reveal a tear like opening on the underside of the glass that does not come through to the top. This is because the upper surface is sufficiently hot to begin slumping while the bottom is just a little too cool. If there is too great a difference, the glass just breaks all the way through.

Also slow heating allows the slump to be accomplished at a lower temperature, leading to fewer problems and to less texture being taken up from the mould.

More detailed information is available in the e-book: Low Temperature Kilnforming.

Stress Testing

You should be testing for stress in any new set up. This includes new processes, different layering, different colour combinations, and any other variation that you make in your basic processes.

You can buy kits called stressometers. The devices called stressometer are not actually meters. They are battery powered light sources with two pieces of polarized film in frames. This is very good for small kiln formed pieces. For larger pieces you can use your light table with larger pieces of polarized film. A description of how to use these is given here.

Stress appears as a “halo” of light around the stressed areas. The more light that appears in, or at the edges of the piece, the greater stress is being indicated. The amount of acceptable stress is given in a Bullseye Technical Note.

However, if you test only the combination of glass you propose to use, you will not know if the stress is from incompatibility or from annealing – the appearance is the same for both. This means that you need to place an additional test into the kiln, to determine the adequacy of annealing. This is especially important when tack fusing and doing thick work. The process for doing this is given here.

How Annealing Affects Slumping

It is often claimed that inadequate annealing of the fused blank can cause breaks during a slump firing.

If annealing is the cause, it is likely to break on the rise in temperature.  Once the piece has reached the annealing temperature, any breaks will be due to thermal shock on the way down.  An annealing break usually has a hook at both ends of the break, although this is more difficult to determine in a shaped piece.

Thermal shock tends to be along straight(ish) lines, often between thick and thin, or strongly contracting colours.  It tends to happen on the cool down. 

Breaks on the rise or fall in temperature are difficult to distinguish on slumps.  The temperature is low enough that there is little to distinguish the sharpness of the edges.  The real method of determining, is to try to fit the pieces together.  If they fit exactly, the break was during the cooling.  If they have even a little variation in fit, the break occurred on the rise in temperature.

If the annealing of the slump is marginally inadequate, it may break hours, days, weeks after cool.  The less stress the longer it will survive.  This will not be the result of any inadequate annealing of the fused blank. Only the last annealing is relevant to the soundness of the piece.

How can you ensure the annealing on a slumped piece is adequate?

You need to check the fused blank for stress before slumping to ensure it has no or very little stress.  The anneal for unstressed items needs to be at least equivalent to, or longer, as for the fused blank.  Testing has shown that annealing for one layer greater than the calculated thickness results is less stress than annealing for the thickest part.

Fire more slowly than usual for blanks with moderate stress to avoid thermal shock during the first ramp.  Anneal the slumped piece for one layer more than was done for the blank.  There is no other requirement for slower annealing.

Pieces with significant stress need to be returned to the kiln to be annealed.  Fire them up significantly more slowly than you normally would for a piece that thick.  This may be one half or less the speed used on the un-fused pieces. Anneal as for a thicker piece.