Fused Glass in Dishwashers

“Can glass be put into dishwashers?”

image credit: very.co.uk

There are many recommendations to avoid placing fused glass into a dishwasher.

The main reasons given are:

·        Corrosion

·        Devitrification

·        Etching and

·        Breaking.

There are distinct differences between these effects.

Corrosion

Glass corrosion generally comes from constant contact with moisture and has a greasy feel.  As experienced by weather or washing, the wetting of glass is not constant, and it dries between wettings.  No visible corrosion is present on window glass and, although float glass is a little different from fused glass, the same effect applies.

Devitrification

Devitrification occurs at much higher temperatures than those created in a dishwasher, and therefore is not a risk.

Etching

The main risk is etching from the washing process.  This can be mechanical or chemical, and dishwashers combine both. Over time, the glass will be etched just the way lead crystal is in a dishwasher.

Breaks

Glass breaks can occur in the dishwasher because of the shock of hot water.  Most dishwashers rinse while heating the water, so the glass experiences only slow rises in temperature.  Float glass of 4mm can withstand 140˚C differentials according to manufacturers.  Full and tack fused glass is not as homogenous as float glass and will be affected by smaller temperature differentials.  So, there is a small risk of breaks in dishwashers.

Additional risks relate to the layup of the glass. 

• ·   Tack fused glass has a variety of thicknesses that make it more prone to breaks from temperature differentials.
• ·   Contrasting colours can react differently and split at the contact lines.
• ·   Large internal bubbles can cause difficulties, which may arise from the insulating element of the contained air, or simply because of thickness.

 

Slumping Splits

 This is a description of the analysis process to determine the possible causes of a split during a slump.     

Credit: Maureen Nolan

Observe the piece.

It is a tack fused piece, about 20cm (8") square, which has been slumped. 

The base layer is of clear. The piece has three additional layers, but the fourth layer is only of small glass dots and rectangles.  The central, heart, area is made of three layers.

A split has appeared during the slump. It is split irregularly through pieces rather than around them.  It is split through the thickness but only partially across the piece.

In one area the (brown) third of four layers spans the split.  Further to the left a brown second layer seems to have broken, but still spans the split.

Threads and particles of glass are connecting across the split. 

The edges are probably sharp, although only so much can be deduced from a description and one photograph.

History of the Piece

The tack fused piece has been put in a mould to form a platter and has split during the slump.

The schedule in essence was:

139ºC/250ºF to 565ºC/1050ºF for a 30’ soak (some pauses but all at a ramp rate of 139ºC/250ºF)

83ºC/150ºF to 688ºC/1270ºF for 10’

222ºC/400ºF to 516ºC /960ºF for 60’

111ºC/200ºF to 427ºC/800ºF for 10’

167ºC/300ºF to 38ºC/100ºF, off

 

The assumption is that the tack fused piece received a similar annealing soak and cool.

 

Diagnosis

Too fast

Slumping a tack fused piece of three layers plus decorative elements on top needs to be fired as for 19mm (6 layers) minimum (twice the actual).  My work for the Low Temperature Kilnforming* eBook showed best results are achieved by slumping as for one more layer (21 mm/0.825” in this case).  This gives a proposed schedule of:  

120ºC/216ºF to 630ºC/1166ºF (not 688ºC/1270ºF) but for 30 to 45 minutes

AFAP (not 400ºF) to anneal 516ºC/960ºF for 3.5 hours (not 1 hour)

20ºC/36ºF to 427ºC/800ºF, 0

36ºC/65ºF to 371ºC/700ºF,0

120ºC/216ºF to room temperature

 
Commentary on the proposed schedule:

The slump is relatively shallow, so a low temperature with a long soak is the most suitable schedule for this piece.  The drop to anneal is at a sedate rate of 222ºC/400ºF.  This is inappropriate, generally.  Just as there is a rapid rate to top temperature to avoid devitrification, so there needs to be an AFAP drop to anneal, also to avoid devitrification.  The anneal soak was not the cause of the break, but it is worthwhile noting the recommended anneal soak and cool rates are longer and slower than that used.  This is a note for the future.

 

Suspect Tack Fuse

If the tack fuse schedule was like the slump schedule, the slump was started with inadequate annealing in the previous firing.  More importantly, the evidence for an inadequate tack fuse is that the split under the brown rectangle was through the clear and red on top, but the split left the brown intact.  This means it was not securely fixed to the red below it. 

 

If the condition of the tack fuse is not sound, it is probable that difficulties will be experienced in the slump.  The poster commented “… why do [these splits] happen only when slumping – it came through tack just fine.”    It is probable the tack fuse was not “just fine”.  The way to be sure the previous firing was just fine, is to test for stress.

 

There is enough clear in this piece that an inspection for stress could be conducted by use of polarising filters before the slump.  Testing for stress is a simple viewing of the piece between two sheets of polarised light filters.  Doing this test will give information on the amount of stress, if any, in the flat tack fused blank.

 

Slump Split

During slumping the glass is subjected to more movement and therefore stress than while being fired flat.  The glass is often only barely out of the brittle zone when being slumped and that makes the stress more evident during the early part of the slump. This requires careful inspection of the failed piece.

 

Look at the glass surrounding the split.  My opinion is that the edges are sharp.  If rounded, the threads of glass from the edges of white would have melted to the edges of the split rather than spanning it. 

 

It appears the top layers were hot enough for less viscous glass on top to form stringers that span the break as the underlying layers split.  It is probable that the split was during the plastic phase of the slump for the upper glass, but  the lower layers were not as hot and suffered thermal shock. 

 

This split of lower layers, while the overlying ones are whole, is often seen in tack fuses, although the top ones do slump into the gap as the firing proceeds.  In a slump there is not enough heat, time or space, for the brown piece to slump into the gap.  Both splits appear to be a result of too rapid firing.  In the flat fusing work, the split results from too fast a ramp rate during the brittle phase of the glass.  But the slumping splits appear to occur after the brittle phase, almost as a slow tear in the glass. This may result from the differential heating of the layers if not fully combined.  It may also indicate the split developed slowly. 

 

One other observation is that these splits seem to be more frequent during the slumping of tack fused pieces.  As speculated above, it may be the inadequate tacking together of the pieces of glass during the first firing, which can form a discontinuity in transmitting heat.  And it may be that the different thicknesses across the tack fused piece allow stress to build from differential heating of the glass.

 

Rates

 

Whichever of these speculative effects may be true, it appears the ramp rates are suspect.  As mentioned elsewhere* (and in Kilnforming Principles and Practice to be published soon), the reasons for these splits are not fully known.  Even microscopic examination by Ted Sawyer has not produced a satisfactory explanation.  The only practical approach that has been successful is to slow the ramp rates.  However, the appearance of these splits is essentially random (with our current understanding), so prevention is difficult.

 

Conclusion

The probable cause of the split in the slump has been that the ramp rates were too fast.  This may have been made worse by the too short anneal soak, and the too fast cool of the tack fused blank.

 

Remedy

There is no practical rescue for this piece.  Prevention in the future is to use ramp rates that are for at least one layer thicker, if it is full fused.  If it is tack fused, firing as for twice the thickest part plus one additional layer is advisable to slow the ramp rates, allowing all the glass to heat and form at the same rate.

 

 

*Low Temperature Kilnforming; an Evidence-Based Approach to Scheduling.  Available from:

Bullseye

and

Etsy

Relative stress in Tack and Full Fused Glass

There is a view that there will be less stress in the glass after a full fuse than a tack fuse firing.

This view may have its origin in the difficulties in getting an adequate anneal of tack fused pieces and the uncritical use of already programmed schedules. There are more difficulties in annealing a tack fused piece than one that has all its elements fully incorporated by a flat fuse. This does not mean that by nature the tack fused piece will include more stress. Only that more care is required.

Simply put, a full fuse has all its components fully incorporated and is almost fully flat, meaning that only one thickness exists.  The annealing can be set for that thickness without difficulty or concern about the adequacy of the anneal due to unevenness, although there are some other factors that affect the annealing such as widely different viscosities, exemplified by black and white.

Tack fused annealing is much more complicated than contour or full fusing.  You need to compensate for the fact that the pieces which are not fully fused tend to react to heat changes in differently, rather than as a single unit.  Square, angled and pointed pieces can accumulate a lot of stress at the points and corners. This needs to be relieved through the lengthening of the annealing process.

The uneven levels need to be taken into consideration too.  Glass is an inefficient conductor of heat and uneven layers need longer for the temperature to be equal throughout the piece.  The overlying layers shade the heat from the lower layers, making for an uneven temperature distribution across the lower layer.

The degree of tack has a significant effect on annealing too.  The less incorporated the tacked glass is, the greater care is needed in the anneal soak and cool.  This is because the less strong the tack, the more the individual pieces react separately, although they are joined at the edges.

If you have taken all these factors into account, there will be no difference in the amount of stress in a flat fused piece and a tack fused one.  The only time you will get more stress in tack fused pieces is when the annealing is inadequate (assuming compatible glass is being used).

More information is given on these factors and how to deal with them in this post on annealing tack fused glass and in the eBook Low Temperature Kilnforming available from Bullseye and Etsy.

Revised 5.1.25

Slump Point Test

At a time when we are all going to be trying a variety of glass of unknown compositions to reduce costs of kiln working, the knowledge of how to determine the slump point temperature (normally called the softening point in the glass manufacturing circles) and the approximate annealing temperature becomes more important.  The slump point test can be used to determine both the slumping point and the annealing soak temperature.  This was required when the manufacturers did not publish the information, and it continues to be useful for untested glasses.

The method requires the suspension at a defined height of a strip of glass, the inclusion of an annealing test, and the interruption of the schedule to enter the calculated annealing soak temperature.

A strip of 3 mm transparent glass is required.  This does not mean that it has to be clear, but remember that dark glass absorbs heat differently from clear or lightly tinted glass. The CoE characteristics given are normally those of the clear glass for the fusing line concerned.  The strip should be 305 mm x 25 mm.  

Suspend the strip 25 mm above the shelf, leaving a span of 275 mm. This can be done with kiln brick cut to size, kiln furniture, or a stack of fibre paper.   Make sure you coat any kiln furniture with kiln wash to keep the glass from sticking.

The 305mm strip suspended 25mm above the shelf with kiln furniture.

Place some kiln furniture on top of the glass where it is suspended to keep the strip from sliding off the support at each end. Place a piece of wire under the centre of this span to make observation of the point that the glass touches down to the shelf easier.

The strip held down by placing kiln furniture on top of the glass, anchoring it in place while the glass slumps.

If you are testing bottles, you may find it more difficult to get such a long strip.  My suggestion is that you cut a bottle on a tile saw to give you a 25 mm strip through the length of the bottle.  Do not worry about the curves, extra thickness, etc.  Put the strip in the kiln and take it to about 740C to flatten it. Reduce the temperature to about 520C to soak there for 20 minutes.  Then turn the kiln off.  

Also add a two layer stack of the transparent glass near the suspended strip of glass to act as a check on whether the annealing soak temperature is correct. This stack should be of two pieces about 100 mm square. If you are testing bottles, a flattened side will provide about the same thickness.  This process provides a check on the annealing temperature you choose to use.  If the calculated temperature is correct there should be little if any stress showing in the fired piece.

The completed test set up with an annealing test and wire set at the midpoint of the suspended glass to help with determining when the glass touches down.

The schedule will need to be a bit of guess work.  The reasons for the suggested temperatures are given after this sample initial schedule which needs to be modified during the firing.

In Celsius
Ramp 1: 200C per hour to 500C, no soak
Ramp 2: 50C per hour to 720C, no soak
Ramp 3: 300C per hour to 815C or 835C, 10 minute soak
Ramp 4: 9999 to 520C, 30 minute soak
Ramp 5: 80C per hour to 370C, no soak
Ramp 6: off.

In Fahrenheit

Ramp 1: 360F per hour to 932F, no soak
Ramp 2: 90F per hour to 1328F, no soak
Ramp 3: 540F per hour to 1500F or 1535FC, 10 minute soak
Ramp 4: 9999 to 968F, 30 minute soak
Ramp 5: 144F per hour to 700F, no soak
Ramp 6: off.

Fire at the moderate rate initially, and then at 50C/90Fper hour until the strip touches down. This is to be able to accurately record the touch down temperature.  If you fire quickly, the glass temperature will be much less than the air temperature that the pyrometer measures.  Firing slowly allows the glass to be nearly the same temperature as the air.  

Observe the progress of the firing frequently from 500C/932F onward.  If it is float or bottle glass you are testing you can start observing from about 580C. Record the temperature when the middle of the glass strip touches the shelf. The wire at the centre of the span will help you determine when the glass touches down.  This touch down temperature is the slump point of your glass.  You now know the temperature to use for gentle slumps with a half hour soak.  More angular slumps will require a higher temperature or much more time.

Once you have recorded the slump point temperature, you can skip to the next ramp (the fast ramp 3).  This is to proceed to a full fuse for soda lime glasses. Going beyond tack fusing temperatures is advisable, as tack fuses are much more difficult to anneal and so may give an inaccurate assessment of the annealing. Most glasses, except float, bottles and borosillicate will be fully fused by 815C. If it is float, bottles or borosilicate that you are testing, try 835C. If it is a lead bearing glass, lower temperatures than the soda lime glass should be used. In all these cases observation at the top temperature will tell you if you have reached the full fuse temperature. If not add more time or more heat to get the degree of fuse desired.

While the kiln is heating toward the top temperature you can do the arithmetic to determine the annealing point.  To do this, subtract 40C/72F from the recorded touch down temperature to obtain an approximate upper annealing point.  The annealing point will be 33C/60F below the upper point.  This is approximate as the touch down temperature is, by the nature of the observation. approximate.  

The next operation is to set this as the annealing soak temperature in the controller. This will be the point at which it usually possible to interrupt the schedule and change the temperature for the annealing soak that you guessed at previously. Sometimes though, you need to turn the controller off and reset the new program.  Most times the numbers from the last firing are retained, so that all you need to do is to change the annealing soak temperature.

The annealing soak should be for 60 minutes to ensure an adequate anneal. This may be excessive for 3 mm glass, but as the anneal test is for 6 mm, the longer soak is advisable. The annealing cool should be 83C/hr down to 370C. This is a moderate rate which will help to ensure the annealing is done properly. The kiln can be turned off at that temperature, as the cooling of the kiln will be slow enough to avoid any thermal shock to the annealing test piece.

When cooled, check the stack for stress. This is done by using two polarised light filters. See here for the method. 

Squares of glass showing different levels of stress from virtually none to severe
 (no light emanating for no stress to strong light from the corners indicating a high degree of stress.)

If the anneal test piece is stressed, there could be a number of reasons for the inadequate annealing. It could be that the glass has devitrified so much that it is not possible to fuse this glass at all. If you also test the suspended strip for stresses and there is very little or none, it is evidence that you can kiln form single layers of this glass. You now know the slumping temperature and a suitable annealing temperature and soak for it, even though fusing this glass is not going to be successful.

Other reasons for stress due to inadequate annealing could be that the observations or calculations were incorrect.  

• Of course, before doing any other work, you should check your arithmetic to ensure the calculations have been done correctly. I'm sure you did, but it is necessary to check.  If they are not accurate, all the following work will be fruitless.

• The observation of the touch down of the suspended strip can vary by quite a bit - maybe up to 15C.  To check this, you can put other annealing test pieces in the kiln.  This will require multiple firings using temperatures in a range from 10C/18F above to 10C/18F below your calculated annealing soak temperature to find an appropriate annealing soak temperature.

• If stress is still showing in the test pieces after all these tests, you can conduct a slump point test on a strip of glass for which there are known properties. This will show you the look of the glass that has just reached touch down point as you know it will happen at 73C above the published annealing point.  You can then apply this experience to a new observation of the test glass. 

Revised 30.12.24

Wire in glass

 

The cracks around the wire imbedded in the glass in the above image are not incompatibility cracks. They do not surround the square piece that traps the wire into the glass. These are from differential expansion/contraction stress between the wire and the glass. 

 

Picture credit: Charmaine Maw

This picture shows the stress that a single strand of wire will induce in glass (the bright light around the wire).  Wire is never going to have similar characteristics to the glass, so the glass must be strong enough to contain the resulting stress.  Anything that increases the mass of the wire, such as twisting or spirals, will increase the stress. 

 

Kanthal and nichrome wires are best as included wire hangers. They are designed for high temperature work and so do not weaken from the heat. This means that high temperature wire as thin as 0.5mm/22 gauge can hold a lot of weight.  Much greater weight than is used in most glass objects to be hung rather than fixed.

Keep the wire as a single strand and as thin as possible, consistent with sufficient strength.  Hammering wire flat can also help reduce the stress by thinning it.

Profile

A sharp tacked piece needs to be fired as though thicker. This example is a single layer base and a square of glass to trap the wire fired to a sharp tack.  It needs to be fired as though 2.5 times the thickest part - 15mm.  A rounded tack fuse of the same layup would need to be fired as for 12mm.

Layup

The use of wire in glass needs to consider how the air will escape from around the wire.  Yes, if the wire exits the glass, there is a channel for it to dissipate.  But air tends to collect along the length of the wire.  If the wire is fully enclosed in the glass, the layup must accommodate the need for air escape routes.  This might be with a fine layer of powder, design elements, chips of glass to hold the outer edges of the glass up for longer, or other devices.

 

Scheduling

The example shown at the start of this blog, is a sharp tack and needed the 2.5 times scheduling.  That probably would have avoided the crack in the single layer base.  That single layer cools faster than the wire with the added piece of glass.  A bubble squeeze is a good idea, even though it would not normally be considered.  This gives the best chance of reducing the bubbles that form around the inclusion.

 

You need to be careful about increasing the ramp rate until the glass has passed out of the brittle phase.  This is about 540˚C/1005˚F. The increase in the ramp rate during the brittle phase may cause cracks. It is, of course, more likely to occur during cooling because the metal will be contracting more than the glass during the brittle phase.  This contrast in contraction rates induces stress that may be great enough to crack or break the glass.

 

 

Tack Fused Drops

Description of the piece

The enquirer wants to cover some blemishes on the flat blank with clear powder and also tack fuse some additional pieces to a blank to be used for a vase drop.

Reactions

To avoid the grey appearance that often comes from clear powder at lower temperatures, you need to fire to contour fuse at minimum. 

Outside of the requirement for a contour fuse, my experience of making a drop vase with a tack fused blank shows disappointing results.  The temperature used in drops is not high enough to flatten the tack fused pieces.  During the drop formation, the space between the pieces stretches more than the thicker tack fused areas. The thinner glass becomes hot quicker than the thicker areas.

This leads to occasional stretched holes between the tack fused pieces.  The tack fused pieces appear as protrusions above the surface whether inside or outside.  Unless planned very carefully, these elements can be ugly. They will maintain much of their original shape, contrasting with the surrounding stretched imagery.

 

Recommendation

Put the piece back in the kiln and take to a full fuse, or at the very least a contour fuse. This will enable all the glass to stretch as one in the drop, because of nearly equal thickness.  Nearly even thickness is needed to avoid stretching some areas too thin in relation to the rest of the drop surfaces.

Slower Ramps on Additional Firings

"Every time you fire a previously fired piece you need to slow down."

This is not accurate. If you have not changed anything significant, the annealing does not need to be extended.  The clearest example is a fire polish.  Nothing has been added. The physics and chemistry of the piece have not changed.  If only confetti or a thin frit/powder layer is added, nothing significant for scheduling has been added.  As nothing significant has changes the annealing used in the previous firing can be used again.

Of course, you do need to slow the ramp up rates on the second firing.  This is because you are firing a single thicker piece.  On the first firing, the pieces are individual and can withstand slightly faster rates. But on third and subsequent firings, if nothing significant has been changed, there is no need to slow rates further.

There is a post which describes this further.

"When adding more thickness more time is needed."

This is the occasion when the annealing soak needs to be extended.  Placing a full sheet of clear glass on the bottom, or less usually, the top, and taken to a full fuse, requires slower ramp rates.   The annealing time for a full fuse can be taken directly from the annealing tables for thick slabs.  

The fusing profile for any additional items has a strong affect on the length of the annealing soak.  If the glass is now of uneven thicknesses, and greater care in assigning ramp rates is needed.  The profile for the piece also determines the amount of additional annealing time required.  A sharp tack of a single additional layer will require annealing as for 2.5 times the total height of the piece at the start or the firing. A rounded tack will need two times and a contour fuse will require 1.5 times.  A full fuse can be carried out for the new total height of the piece without any multiplying factors.

 If the intention with multiple firings is to achieve a variety of profiles within one piece, a slightly different approach is required.  A blog post here describes the process.

The general approach to multiple firings is that unless there are changes to the thickness or profile of the glass, no changes in annealing time is required.  

 

Longer Soak or Higher Temperature?

 ‘Is it better to extend the soak or add more firing time when the firing program isn’t quite enough? What are the meanings of “soak,” “hold,” “ramp,” “working temperature” and “top temperature”?’  

Let’s start with some of the terms.

“Soak” and “hold” have the same meaning in scheduling.  Schedules are made up of a series of linked segments.  Each segment contains a rate, temperature, and time.  The time is often called a “hold” in the schedules.  That time can have several effects.  It can allow enough time for a process, such as slumping, to be accomplished.

Although “soak” is entered into the schedules in the same way as a hold, it has a different concept behind it.  The hold when used as a soak allows the set temperature to permeate the whole thickness of the glass.  An example is in annealing. An annealing hold/soak is set.  This is to allow the glass to become the same temperature throughout. 

The ramp is the rate at which the controller is set to increase/decrease the temperature.  This is normally the first element in the segment.

“Top” and “Working” temperature are the same thing.  It is the temperature at which the desired effect is achieved.  They have slightly different nuances.  Top temperature is normally considered as a point where the desired profile will be achieved in a few minutes.  The working temperature is also that, but includes the idea that it will take time for the effect to be achieved.

Which should you alter first – soak time or temperature?

Most important is that you alter only one at a time.  If you alter the two elements at the same time, you do not know which was the cause of the result.

In general, you lengthen the soak if the effect is not achieved at the temperature and in the time set.  There are two reasons for this.  Glass has fewer problems at lower temperatures.  Secondly, the controllers are set up in such a way that it is easy to extend the time. Check your manual for the key sequence to extend the time.  It is more difficult to alter the temperature during a firing. 

To determine if you need more time, you peek into the kiln as the kiln approaches the top temperature.  If the profile has not been achieved by the time set at your working temperature, you enter the combination of keys to keep the kiln at the top temperature until you see the effect you want.  Then enter the combination of keys to skip to the next segment.

Whether you alter time or temperature, depends on what you are doing.  Soak plus temperature equal heat work.  With heat work you can accomplish the same effect at lower temperatures.  It may be that taking more time (usually slower ramp rates) to get to the same or lower temperature, will give the results desired.

For slumping, draping and other low temperature processes extending the hold/soak is appropriate. It reduces the amount of marking that is created by the mould or surface supporting the glass.

When tack, contour, or full fusing, you should be aiming to finish the work in about 10 minutes. Soaking/holding significantly longer increases the risk of devitrification.

For high temperature processes such as pot and screen melts and some flows, increasing the temperature is probably the right thing to do, to avoid the devitrification possibilities of long holds of open face high temperature work.

These can only be guidelines.  Your instincts and experience will help you determine which is the right thing to do in the circumstances.

 

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.

Slumping and Annealing bottles

"Can a tack fuse schedule for fusing glass can be used to slump bottles?"

It may be that this person does not have the confidence to write a new schedule.  They may wish to use an existing schedule for another purpose. The short answer is “Although a Bullseye or Oceanside tack fuse temperature will be high enough to slump bottles, they are not suitable for annealing”.  There are reasons for this. 

The softening point of float glass, which is similar to bottle glass, is 720ºC/1330ºF.  Slumping would normally be done at about 20ºC/36ºF above this. You also need a slumping hold at this temperature much longer than a tack fuse schedule would use.

if you use a tack fuse schedule for a fusing glass, your annealing will be inadequate. Bottle and float glass tend to have an annealing point of around 540ºC/1005ºF. An annealing for fusing glass will be between 515ºC/960ºF and 482ºC/900ºF.  This is likely to be too low an annealing point for bottles.  Also, the annealing soak is likely to be too short. Slumped bottles are very thick at the base where it folds over the cylinder of the bottle.  This requires a longer anneal soak and slower cool than a schedule for a tack fuse of fusing glass.

Checking for stress in the completed work is normal.  It is essential for your finished bottle if you use a tack fuse to fire it.

 

Schedules should be devised for the glass and layup of each piece. Transferring a schedule for fusing to bottle glass is unlikely to be successful.

Slumping Schedules

 When slumping fired pieces, it is most often appropriate to use a slow ramp rate to avoid too rapid expansion of the glass that might lead to a break. Most glass breaks on the ramp up are below 300°C/573°F. It is in this range that there is a rapid expansion of ceramic. This means a slow rate is protective for both glass and ceramic moulds.

Slumping Schedules by Profile (Celsius)

Flat Fuse and Contour Tack

Actual thickness	Ramp 1 rate to 260°C	Soak time (min)	Ramp 2 rate	Slumping  temp. for mould *	Soak time (min)	Anneal as for contour:
6	240	20	240		30	9mm

Rounded Tack

Actual thickness	Ramp 1 rate to 260°C	Soak time (min)	Ramp 2 rate	Slumping  temp. for mould *	Soak time (min)	Anneal as for round tack:
6	150	20	150		30	9mm

Sharp Tack

Actual thickness	Ramp 1 rate to 260°C	Soak time (min)	Ramp 2 rate	Slumping  temp. for mould *	Soak time (min)	Anneal as for sharp tack:
6	120	20	120		30	9mm

 

Slumping Schedules by Profile (Fahrenheit)

Flat Fuse and Contour Tack

Actual thickness	Ramp 1 rate to 500°F	Soak time (min)	Ramp 2 rate	Slumping  temp. for mould *	Soak time (min)	Anneal as for:
0.250”	432	20	540		30	0.375”

Rounded Tack

Actual thickness	Ramp 1 rate to 500°F	Soak time (min)	Ramp 2 rate	Slumping  temp. for mould *	Soak time (min)	Anneal for:
0.250”	270	20	270		30	0.375”

Sharp Tack

Actual thickness	Ramp 1 rate to 500°F	Soak time (min)	Ramp 2 rate	Slumping  temp. for mould *	Soak time (min)	Anneal for:
0.250”	216	20	216		30	0.375”

 

*Of course, the slumping temperature will be altered for the glass according to the manufacturer’s stated range. The nature of the mould will also have a big effect on temperature and time. The soak times at the slump soak are those appropriate for the mould. The annealing soaks are related to the profile of the glass.

Rates

It is most often best to use a slow ramp rate to at least 500°C/933°F. This avoids the risk of inducing a too rapid differential expansion within the glass as it heats up. Experiments relating to the first ramp rate have shown firing as for two layers thicker than indicated by the profile schedule provides the best results. It is then possible to increase the rate as determined by the profile schedule.

The rates for the anneal soak and cool are those that are one layer thicker than determined by the schedule for the profile. This has been shown by experimentation to give the best annealing result – i.e., least stress.

Temperatures

The slumping temperature needs to be altered for two factors:

• ·        the glass according to the manufacturer’s stated range, and
• ·        the nature of the mould.

Many manufacturers are giving recommended temperatures and times for slumping in their moulds. An example is the Bullseye “Quick Tip” which gives suggested temperatures and times for various sizes and natures of moulds that can form the basis for scheduling of slumps. The rates are normally for flat uniformly thick pieces. This will need alteration for tack profile pieces.

Take note of the soak time in these recommendations. If it is less than 10 minutes, it is possible to reduce the temperature by about 10°C/18°F by using a 30-minute soak. This will reduce marking on the back of the glass.

Soaks / Holds

Slumping schedules tend to be more difficult to devise than many other operations in kilnforming because of variations in moulds and what is placed on them. This, consequently, makes observation of the slump more important. It is needed from a point below the target temperature – say 20°C/36°F – to ensure the slump is stopped when it is complete.  If it is not complete, the soak can be extended. The controller manual will give the information on how to do these operations. In general, you schedule slower ramp rates for thicker pieces in combination with the half hour soak. This means for each thickness greater than 6mm, the top temperature can be reduced and still achieve a full slump.

The schedules here are applicable for pieces up to 9mm actual thickness.

Slumping of thicker pieces needs to apply the underlying scheduling method:

• ·        Apply the rate for two layers thicker for the advance to 260°C/500°F.
• ·        Increase the rate after that to one for a single layer thicker than calculated all the way up to the slumping temperature.
• ·        For annealing, also select the rates and times for one layer thicker than indicated by the profile.

 

For example:

• ·        Rounded Tack of Bullseye, 12mm/0.5” thickness
• ·        Schedule for 25mm/1” (2 times multiplier)
• ·        Initial ramp rate for 31mm/1.25” (two thickness greater)

Celsius schedule for up to 9mm actual thickness:

Segment >	1	2	3	4	5	6	7
Rate	150	150	ASAP	15	27	90	off
Temp	260	Top	482	427	370	RT
Time(mins)	20	30	240	0	0	0

and in Fahrenheit:

Segment >	1	2	3	4	5	6	7
Rate	270	270	ASAP	27	49	162	off
Temp	500	Top	900	800	700	RT
Time(mins)	20	30	240	0	0	0

 

A further example:

• ·        Sharp Tack of Bullseye, 0.5” thickness
• ·        Schedule for 31mm/1.25” (2.5 times multiplier)
• ·        Initial ramp rate for 38mm/1.5” (two thickness greater)

 Celsius schedule for up to 9mm actual thickness:

Segment >	1	2	3	4	5	6	7
Rate	78	78	ASAP	11	20	65	off
Temp	260	Top	482	427	370	RT
Time(mins)	20	30	300	0	0	0

and in Fahrenheit:

Segment >	1	2	3	4	5	6	7
Rate	140	140	ASAP	20	36	117	off
Temp	500	Top	900	800	700	RT
Time(mins)	20	30	300	0	0	0

 

These examples show that considerable differences in scheduling are needed for different tack profiles. It also shows longer annealing soaks and slower cooling rates are required for sharp than rounded tack pieces.

 

More information is given in the e-Book Low Temperature Kilnforming. and at Bullseye eBooks