Changing size in Slumping

 “I have full fused a single piece of glass with a few small pieces on top.  I thought it would shrink some as I had been told, but it maintained its size and still fit the mold for slumping.” 

I believe the enquirer is talking about a single layer circle changing size at full fuse.  Dog boning is much less evident in circles than rectangles.  The glass retreats evenly all along the edge.  This gives the appearance of retreating less than rectangles.  The absence of any big change in size may also result from thinning of the centre.  The amount of size change will be affected by the temperature of the full fuse too.  In this case there were additions which will have resisted any tendency to shrink.

Lower top temperatures, more rapid ramp rates to the top, and shorter holds will have the effect of limiting the movement of glass toward 6mm thick.

credit: Bullseye Glass Co

The viscosity of glass at full fuse is enough for it to attempt to pull up to 6mm. At casting temperatures, the viscosity is so low that 6mm of glass spreads out.  Temperature affects viscosity.

 

At slumping temperatures (620˚C - 680˚C / ca.1150˚F - 1260˚F), the viscosity high enough that the dimensions of a circle do not change. A circular piece of 3mm glass held at slumping temperatures does not change dimension.  It may, if held long enough take on a kind of satin sheen, rather than a fire polish.  But the viscosity  is low enough to allow the glass to form to the mould, given sufficient time. The resulting slumped piece will appear to be smaller than the mould. If you measure the piece around its outside curve, you will find the distance is almost the same as the diameter of the blank. 

 

Changing size on a single layer piece is dependent on the temperature and heat work applied to the piece.

Kilnforming with 3mm Glass

 A power point presentation I made a few months ago to the group Lunch with a Glass Artist.

It is 33 slides long.

Kilnforming with 3mm Glass.pptx

When to Open a Cooling Kiln

Credit: Glass House Store

Questions about when it is possible to open the kiln during the cool down to avoid thermal shock get the answer, “it depends….”

These dependent variables include:

Temperature Differentials

Thermal shock is related to how quickly a piece can cool without developing stress that cannot be contained within the piece.  So, when the temperature differential is a few tens of degrees between room and kiln air temperature it is less risky than when the difference is hundreds of degrees.

This means that there is a relation between room temperature and when you can open the kiln safely.  If the room is at sub-zero temperatures, you will need to wait for a lower temperature in the kiln, so the temperature differentials are no greater than when the room is warm.  Remember the glass can be much hotter than the air that the thermocouple measures.

Cooling rate of the kiln

The natural cooling rate of the kiln (that is, in the unpowered state) will affect when you open.  If your kiln cools very slowly from 150°C, you may feel confident to open the kiln a little to speed the cooling from that temperature.  If you kiln cools quickly - usually in smaller kilns - then you need to wait longer for a lower temperature to be achieved.

Size of the piece

The size of the piece(s) relative to the kiln size has a bearing on when it is safe to open the kiln to speed cooling.  The more space the piece takes up in the kiln the cooler the temperature reading needs to be before you open the kiln.

Placing

The placing of the glass has an affect too.  If the glass is at the front of a front opening or top hat kiln, it will cool more quickly and unevenly than one at the back. A large piece placed more to one edge than another will also require lower temperatures before opening.

Thickness

The thickness of the glass also needs consideration.  The thicker the glass, the hotter it will be in relation to the measured air temperature, and so the longer it needs to be left to cool before opening.

Type of kiln

Your kiln may cool slowly or quickly, but the style of the kiln is important too.  The kiln may be brick lined or fibre lined, or a combination.  The greater the mass of the insulation, the earlier you can open, as the dense brick will radiate heat back toward the glass.

If you have a top hat kiln it is probable that you can open earlier than if you have a top opening or front door opening kiln, as they will dump hot air slower than top and front opening kilns.

The venting method

The way you open the kiln to increase the cooling rate is important.  If you open vents, that provides a gentler flow of cooler air than opening the lid or door.  If you open lids or doors, you need to wait for a lower temperature than for opening vents.

And I am sure there are other considerations.  But these are enough to show that there is not a single answer.  The answer is in relation to the kiln and its contents.

Acceptable Cooling Rates

The speed of cooling that a glass can sustain is indicated by charts giving the rate of cooling for the final rate of decrease to room temperature.  Faster rates might be induced by turning the kiln off at 370°C and opening the door/lid at some slightly lower temperature.

This means that you need to know how fast a cooling rate is acceptable.  The bullseye research suggests that 300°C per hour for the final cooling is as fast as you would want to cool a 12mm thick piece.  This is in a closed environment.  Therefore, you will want to be slower – at least half the speed for a partially opened kiln of say 5cm. 

My predictions for acceptable cooling rates are (with a room temperature of 20°C; a piece evenly thick and 30cm square, but less than half the area of the kiln floor; and a top hat kiln):

6mm -   300°C per hour (although I never use more than 200°C per hour)

12mm - 150°C per hour

19mm - 75°C per hour

25mm – 45°C per hour

Note: Tack fused items with these total heights need to have these rates halved, or use the rate suitable for a piece twice the thickest part.

But!

You cannot open the kiln until the natural cooling rate is at the predicted acceptable rate of cooling or less, to be safe.

The natural cooling rate at various temperatures can be determined by observing temperature falls in relation to time intervals between those observations.  You can make a chart to indicate the cooling rate at different temperatures.  The kiln will naturally cool more slowly at lower temperatures. 

Schedule to room temperature

A protection against too rapid cooling is programming to room temperature.  If your kiln is cooling less rapidly than you predict is acceptable, you are using no electricity – OK, maybe a tiny fraction of a kilowatt to keep the controller operating. But there is no worry of using excess electricity.

The point of programming to room temperature is that if the air temperature in the kiln cools faster than predicted, the controller will turn the kiln on.  You will need to be present for a while after venting the kiln to hear if it turns on and you can lower the lid to a point where the kiln does not turn on, indicating the rate of cooling is less than put into the schedule.

An example:

Assume you predict that 150°C per hour is the appropriate rate of cooling from 370°C. Also assume you open the kiln at 100°C and a minute or so later you hear the kiln start.  Then you know that you have opened the kiln too far causing a more rapid cooling than 150°C per hour and you need to close the opening to less than the current state.  This probably will be a progressive thing.  You will come back, say, half an hour later and open a little more.  Everything seems fine, but 10 minutes later you hear the kiln switch on again.  Oops! You opened too much – you need to close the kiln a little.  This may repeat several times.

The real answer to when you can open your cooling kiln is dependent on many variables.  You will have to decide on how critical these are in relation to the piece(s) you have in the kiln.  Once you have decided on the appropriate rate, you should program that into your schedule for the final segment.  This means when you partially or fully open the kiln the controller will switch the kiln on when the cooling rate is faster than you wanted.

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.

Breaking Apart the Last Two Thin Strips

For multiple thin strips of even widths, score all the strips first. Then break all the scored strips off the remaining sheet as one piece. Start the breaking process by breaking the scored sheet in the middle, then in the middle again, until there are only two to break apart.

Cut running pliers are most useful until the last two thin strips are to be divided.

At that point use two breaking pliers to hold each side of the two pieces of glass. The noses of the pliers should almost touch on either side of the score line. Apply pressure in a downward pivoting motion to break the pieces apart.

Thin strips

Breaking Tapering Pieces

Breaking thin pieces of glass can be tricky, but there are a few things you can do to help direct the break the way you want it to go.

Relieving scores made alongside curved and tapering pieces make the breaking more certain. A relieving score is one that is in addition to the primary score. This additional score will allow you to break the thin or tapering piece from the larger sheet safely, and then go on to break out the delicate piece.

The object is to always be breaking away less glass than is retained. The use of two breaking/grozing pliers, one on each side of the narrow pieces gives more even pressure than fingers or cut running pliers with wide jaws.

When breaking tapering pieces of glass you should normally grasp the thin end in fingers or pliers and run the score toward the  thick end and ease the run of the score. When the score opens an initial distance, turn the glass end for end and run the score back to the opened one. 

Firing for 3mm Channels

A question has arisen on how to put together a design of pieces for a lamp, but only one layer thick, because 3mm is as thick as the fittings will accept.

The design has no overlaps, so it is a series of butted 3mm thick pieces.  Damming has not been successful in keeping the parts from retreating from one another.  This means that making the design as a single layer will not be successful.

The problem is how to make a two-layer piece that will be able to fit into 3mm fixings.

Design on oversize 3mm base

One way to overcome the fixings’ limitations is to make the bottom layer larger than the top.  The bottom can be any colour you choose.  Make the design on top of that. 

The designed pieces will need to fit snugly beside the fittings. However, the bottom needs to be cut larger than the final size, as it will retreat and become smaller during the firing.  About 20mm larger all around will usually be enough extra for ease of cutting down. If you fire with a larger base piece, you can cut it to size after firing, so it will fit the width of the opening and still fit inside the 3mm fitting space.  This will make your design proud of the fittings.  This may, or may not, be possible for the lamp’s fitting design.

An alternative

Maybe that is not the only way to look at the problem. There is another way.  It is essentially the opposite of the first approach. 

Make the top layer larger than bottom. The design will be on the top still, but with larger than final dimensions to accommodate the reduction in size of the single layer.  The bottom layer will need to be small enough to fit within the space between the fittings of the lamp.

To keep the unsupported parts of the upper layer in one plane, support the larger upper layer with 3mm fibre paper. Coat the fiber paper with boron nitride or cover with powdered kiln wash, Thinfire or Papyros to get a smoother back.  When fired, cut the piece to size.  If you like to score on the smoothest side, you can support the edge with the fibre paper or other 3mm substance.  If you are confident, you can score on the back with no special support.

These are two approaches to making a piece to fit in a 3mm channel.  This will apply to insertions of fused glass leaded glass panels, as the came is designed to accommodate 3 mm glass.

Broken Base Layers

Sometimes in fusing, the base layer can exhibit a crack or break without the upper layers being affected.  This kind of break almost always occurs on the heat up.  In a tack fuse, the top layers may still be horizontal and unaffected by the break beneath them.  If a full fuse, the upper layers will slump into the gap, or apparently seal a crack that is apparent on either side.

An example of tack fused elements on top of a previously fused base

Causes

This is more likely to be seen where there is a large difference between thicknesses.  If the base is a single or double layer and there are several layers of glass – especially opalescent – on top, there is a greater chance for this kind of break to occur.

The reason for this kind of break is that the upper layers insulate the part of the lower layers they are resting upon.  Glass is an insulator, and so a poor conductor of heat.  This means that the glass under the stack is cooler than the part(s) not covered.  A break occurs when the stress of this temperature differential is too great to be contained.

An example of  stacked glass in a tack fusing

This kind of break can also occur when there are strongly contrasting colours or glasses that absorb the heat of fusing at different rates.  One case would be where the dark lower layer(s) were insulated by a stack of white or pale opalescent glass.  The opalescent glass will absorb the heat more slowly than the dark base.  This increases the risk of too great a temperature differential in the base.

Reducing the risk of these breaks.

Even thicknesses

One way to reduce the risk of base layer breaks is to keep the glass nearly the same thickness over the whole of the piece.  Sometimes this will not give you the effect you wish to obtain.

Slow the firing rate

Another way is to slow down the temperature rise.  Some would add in soaks at intervals to allow the glass under the stack to catch up in temperature.  As we know from annealing, glass performs best when the temperature changes are gradual and steady.  Rapid or even moderate rates of advance with soaks, do not provide the steady input of heat.

This prompts the question of how fast the rate of advance should be, and to what temperature. 

Rate of advance

The rate of advance needs to take account of the thickness differential and the total thickness together.  A safe, but conservative, approach is to add the difference in thickness between the thinner and the thickest parts of the piece to the thickest.  Then program a rate of advance to accommodate that thickness.  E.g., a 6mm base with a 9mm stack has a total height of 15mm.  The difference is 9mm which added to 15mm means you want a rate of advance that will accommodate a 24mm piece.

The rate of advance can be estimated from the final annealing cool rate required for that thickness.  In the example above, the rate would be about 100°C per hour.  The more layers there are, the more you need to slow the heat up of the glass. The Bullseye table for Annealing Thick Slabs is the most useful guide here.

Firing already fused elements

If you were adding an already full fused piece of 9mm thick to a 6mm base, you could have a slightly more rapid heat up, bu not by a lot. This is because the heat will be transmitted more quickly through a single solid piece to the base glass.  It is safer to maintain the initial calculation. If your stack is tack fused, this will not apply, as the heat will move more slowly through the layers of the tack fusing much the same way as independent layers on the initial firing.

Conclusion

The general point is that you need to dramatically slow the speed of firing when you have stacked elements on a relatively thin base.  Even a two layer base can exhibit this kind of break when there is a lot of glass stacked on it.

Scheduling Relates to the Piece

My piece cracked, but I've always used this schedule and it has worked.

One schedule is not for all pieces. A number of factors affect the scheduling of a firing.  Some of them are:

Thickness

• The thicker the stack of glass, the slower the advance and anneal should be. 
•  The more layers of glass there are, the slower the rate of advance should be. 
•  The more uneven the thickness, the slower the temperature changes should be.

Angularity

• Glass with right angles or even more acute angles needs slower schedules than round or oval shapes.  

Degree of fuse

•  Laminated and tack fused pieces need much slower annealing and re-firing schedules.  

Contrasting colours

• Pieces with strongly contrasting colours of glass need slowing in heating and annealing.

Size

• To some extent the increased size will need some slowing of the schedule. Size becomes more important as you near the edge of the shelf or nearer to the sides of the kiln. Jewellery scale items can have an accelerated schedule.  

Mould base

• The size and shape of the mould will affect the speed and temperature of the scheduling.         
• The type and style of mould affect the schedule.  Drapes and especially over steel moulds require slower schedules. 

Position in the kiln

• The closer the glass is to the elements whether top or side, the slower the schedule must be.
• The less central on the shelf, the more care must be taken in scheduling.  

Type of kiln

• A kiln constructed for ceramics needs different scheduling considerations than one for fusing.  
• A kiln with side elements needs more careful firing than one with only top elements.

Bubbles in Thin Pieces

Bubbles are often blown through frit castings and other thin pieces. This most results from insufficient volume of glass in the mould or on the shelf. Also the design can induce bubbles where there are thinner parts surrounded by thicker parts. As the glass softens, the surface tension of glass - from around 730 - causes it to pull up to equalise at about 6-7mm thick. This causes thinning in certain areas to allow thickening in other areas. This then leads to the risk of blowing bubbles through the glass where the glass has become thinner.

If thinner work is required, you can fire an over-sized piece to about 750C for a short time and then cut it back to the final size. If you want a flat thin sheet, you can also place the glass between two kiln shelves. You need to separate the shelves with a 3mm spacer to keep the upper shelf from coming completely down on the shelf, giving an extremely thin fragile piece of glass.

Effects of Multiple Layers

Stacking layers of glass fully or partially over the base layer has significant effects on the firing of the whole piece.

Glass is a poor conductor of heat, so you need to be careful to allow the heat to penetrate to the base layer to avoid thermal shock. There also is the effect of the (very small) insulating space between each sheet. The effects of multiple, even layers can be seen from this table based on Graham Stone's* work:

3mm layers

1 sheet – Initial Rate of Advance =1000ºC to 475ºC (less than half an hour)

2 to 3 layers – IRA = 240ºC to 475ºC (ca. 2 hours)

4 layers – IRA = 100ºC to 475ºC (4.75 hours)

6 layers – IRA = 25ºC to 125ºC, then 30ºC to 250ºC, then 40ºC to 375, then 50ºC to 475 before 150C to top temperature (ca. 15.5 hours)

This shows the dramatic effect increasing the number of layers has on the firing schedule to make sure the heat gets to the bottom sheet evenly. If you compare the initial rates of advance (IRA) with the same thickness, but fewer sheets you can see the space between layers is important.

6mm layers

1 sheet – IRA = 320ºC to 475ºC (ca. 1.5 hrs)

2 layers – IRA = 240ºC to 475ºC (ca. 2 hrs compared to 4.75 hrs for 4 layers of 3mm)

3 layers – IRA = 200ºC to 475ºC (ca.2.5 hrs compared to 15.5 hrs for 6 layers of 3mm)

These are the fastest safe firing speeds for evenly covered sheets. 

This difference in firing times for stacks of thicker glass, shows how important it is to fire sections of the stack before the final firing of all the layers together.  It also reduces the risk of bubbles developing within the stack. 

If you are thinking of tack fusing with thicker and thinner areas, you need to take account of the differences in thickness in the various areas of the piece when preparing your schedule. You will need to decrease your IRA by quite a bit. So you might want to be thinking of firing some of your pieces to be added to the base layers before tacking them in an additional firing to reduce the risk of thermal shock to the base layer.

* Firing Schedules for Glass; the Kiln Companion, by Graham Stone, ISBN 0646 39733 8

Cutting thin strips

Cutting thin strips of glass such as used in Mission Style patterns and precision fusing projects requires skill and assistance. For transparent and translucent glass you can arrange a right angle guide on a board and tape a piece of lined notepaper to the jig. Use a cutting square and move it right along the lines on the note paper making four or six scores at a time and then breaking on the last score first and then every other score, and then each one in half.

Another method is to use the edge of the bench as a guide. With a small adjustable carpenter’s square, you hammer in nails at the predetermined width (plus half the thickness of the cutter head). Align the glass to the edge of the bench between the nails. Place a straight edge against the nails and score. This gives strips of the same width every time, but works best with strips of 10mm (3/8”) or more. This is illustrated in the processes section.

The thinner the strips are to be cut, the more important it is to make the scores and then divide the sheet in half - the two halves in half each - the 4 quarters in to halves, etc, until you are down to the piece that only needs to be divided in two.

The thinnest strip that can be cut is a fraction wider than the thickness of the glass.  This is because the glass will always break toward the weakest area.  If the strip is thinner than the glass is thick, it will break within the strip.  The narrowest strip that can practically be cut is at least one or two millimetre wider than the glass is thick.  So, if you have 3mm glass, the narrowest you can cut is 4 or 5 mm.  Four millimetre wide strips can only be cut from really smooth consistent thickness of glass sheets.  It is much more practical with decorative glass to limit the width to twice the thickness of the glass you are cutting.

Dividing two thin strips

Single Layer Firing

Preparing a Single Layer for Further Kiln Work

There can be circumstances where you do want to fire a single layer in building up your project. This is more often difficult on rectangular than round pieces.

Some of the considerations are:

Temperature

Heat work

sizing

Cleaning after firing

Firing a 3 mm piece to anything over a laminated tack fuse normally leads to the edges drawing in creating a “dog bone” effect and often leading to bubbles in the interior at higher fusing temperatures. So one approach is to fire at low temperatures and accept relatively sharp edges on the piece.

Diagram of the full fused results of different thicknesses 

However the concept of heat work can help in this situation. Glass reacts to the accumulation of heat, so that slow advances or long soaks can achieve the desired results at a lower temperature without – in this case – getting the “dog bone” effect. This does require a bit of experimentation. Keep good records of all the stages of experimentation as the effects achieved with various combinations of temperature and time will come in useful later.

It is possible that using the concept of heat work will not be sufficient to achieve the desired results. Then you need to consider placing your design in the centre of a larger piece. Fire this to the lowest possible temperature to achieve your results and then cut the fired piece to size. You will need to fire polish or cold work the edges to get a suitable finish on the edges.

The central white piece shows the results of single layer firing that could be altered by the above technique

If you are going to re-fire any of these single-layer pieces, you need to clean them very well. Any dust or other contamination will be incorporated into the final piece. This is especially true if you are combining a flip and fire technique with this single-layer firing.

Cutting Wheel Angles

These are the wheel angles recommended by The Fletcher-Terry Company for various glasses:

114 to 134 degrees – 2mm float glass
130 to 140 degrees – 4mm float glass
134 to 140 degrees – 3mm to 6mm float glass
148 to 154 degrees – 12mm to 25mm float glass
134 to 140 degrees – stained glass
88 to 114degrees – borosilicate glass

http://www.fletcherviscom.com/home.shtml