Sample Tiles

credit: Tia Murphy

There are advocates for making tiles as references for future work.  

• They show the profiles achieved at different temperatures.  
• They can be stored for easy visual reference when planning a firing.  
• It is a useful practice for any kiln new to the user.  

These tiles are assembled in identical ways to enable comparisons.  They should include black and white, iridised pieces- up and down, transparent and opal, and optionally stringers, confetti, millefiori, frit and enamels.  

The tiles are fired at different top temperatures with the same heat up schedule with the top temperature of each at about 10C or 20F intervals.  These show what effect different temperatures give.  Start the temperature intervals at about 720C or 1330F.

This is a good practice, even if time consuming.  It gets you familiar with your kiln and its operation.  It gives a reference for the profiles that are achieved with different temperatures at the rates used.

Ramp rate and time

But, as with many things in kilnforming, it is a little more complicated.  The effect you achieve is affected by rate and time used as well as the temperature.

The firing rate is almost as important as the temperature.  

• A slow rate to the same top temperature will give a different result than a fast rate.  
• The amount of heat work put into the glass will affect the temperature required.  
• Slow rates increase the time available for the glass to absorb the heat.  
• Glass absorbs heat slowly, so the longer the time used by slower rates, the rounder the profile will be.

Since time is a significant factor in achieving a given profile, any soaks/holds in the schedule will affect the profile at a set temperature.  A schedule without a bubble squeeze will give a different result than one with a bubble squeeze at the same temperature.

To help achieve knowledge of the rate/time effect, make some further test tiles.  Use different rates and soaks for the test tiles of the same nature as the first temperature tests. But vary only one of those factors at a time. Consider the results of these tests when writing the schedule for more complex or thicker layups. 

Mass

Also be aware that more mass takes longer to achieve the same profile.  Slower rates and longer times will help to achieve the desired profile at a lower temperature.  It is probably not practical to make a whole series of test tiles for thicker items.  But, a sample or two of different thicknesses and mass will be helpful to give a guide to the amount of adjustment required to achieve the desired outcome.

The results of sample tiles are due to more than just temperature.  They are a combination of rate, time, and temperature (and sometimes mass).  These factors need to be considered when devising or evaluating a schedule, because without considering those factors, it is not possible to accurately evaluate the relevance of a suggested top temperature.

See also: Low Temperature Kilnforming, available from Bullseye and Etsy

Scheduling for Thick Landscapes

Thick slabs often involve numerous firings of increasingly thick work.  I am using an existing example, with their permission, of the first stages of a thick landscape.  The initial concern was with bubbles in the first layup, then the strategy for firing the thick slab.

Plans

This is the first part of a landscape with depth.  It will be fired 5-7 more times.  This first piece will be inverted for the next firing with the clear facing up, to avoid reactions between the colours.  It is similar to an open face casting. There is a Bullseye Tip Sheet on open face casting that will give a lot of information.

Layup

Picture credit: Osnat Menshes

This work has a base of clear that is mostly overlaid with one layer of 3mm pieces, although in some places another layer, and there are some pre-fired elements as well.  It is fired on Thinfire shelf paper.

Bubbles 

There is concern about the number and size of the bubbles after the firing, and how to avoid them.  Will they grow over the multiple firings?

The many small bubbles are characteristic of kilnformed glass.  The few larger bubbles may result from the frit that is under the pieces that form the top surface.  And there are some overlaps of clear over colour that may form pockets where air can collect. I advise leaving the scattering of the frit until all the decorative pieces are in place.  The bubbles will migrate toward the top during the multiple firings.  They will not grow in size unless they combine during the upward migration.  A later suggestion about reducing the number of firings will reduce the bubble migration and risk of increasing in size.

Picture credit: Osnat Menshes

Schedule

Proposed Schedule (Temperatures in degrees Celsius)

1: 180 – 560, 30’    I would go to 610 for 30'

2: 25 – 680, 120’    I would use only 30'

3: 220 – 810, 15’    I would set the top temperature at 816, 15’.

4: 9999 – 593, 30’  Eliminate this segment. 

5: 9999 – 482, 120’ I suggest one hour soak

8: 55 – 370, off      83 – 427, 0’

7: 150 – 371, 0’

8: 330 – to room temperature, off.

 

Eliminate segment number 4.  Any temperature equalisation done at this temperature, is undone by the AFAP to the  annealing.  The temperature equalisation occurs at the annealing temperature. No soak at an intermediate temperature is required.  This blog post gives some information about annealing above and below the annealing point (Tg). 

Firing Incremental Layers

The plan is for five to seven more firings.  Continuing to build up the thickness on each firing, may have some problems.

• There is increased risk of compatibility problems when firing a piece to full fuse many times.
• There is a risk of more bubbles and of the existing ones becoming larger as they move upwards and combine with other smaller ones.
• With each firing the thickness is increasing and so becoming a longer firing.  This is because the heat up, annealing, and cooling each need to be longer.  For example - 6mm needs 3hour cooling, 12mm needs 5 hours, 19mm needs 9 hours. 

Multiple Slabs

These are the main reasons that I recommend firing a series of 6mm slabs separately and combining them in one final firing.  Firing a series of 6mm slabs and then combining them in a single long and slow final firing has advantages.

• The individual pieces do not need to go through so many full fuse firings, reducing the risk of compatibility problems.
• The small bubbles in each firing will not have the chance to rise through all the layers to become larger.
• The total time in the kiln for the combined pieces will be less than adding layers to already fired layers.

Examples

It is often difficult to convince people that firing by adding incrementally to an existing slab, longer firing times are required than by firing a group of 6mm slabs and a single combined firing of all the slabs.  I give an example to illustrate the differences.

Annealing

Assume there are to be a total of eight firings (existing 6mm slab and 3mm for each of seven more firings).  Also assume that each additional firing is of 3mm. This makes a total of 28mm.  Compare annealing and cooling times for each firing:

Firing      thickness       anneal and cool (hours minimum)

1            6mm                    3

2            9mm                    4

3            12mm                   5

4            15mm                   7

5            18mm                   9

6            21mm                   11.5

7            25mm                   14

8            28mm                   17

Total                                   70.5 hours annealing time (minimum)

To fire up 5 six millimetre slabs takes less time – 3 hours annealing and cooling time for each firing cumulates to 15 hours.  Add to that the final firing of 17 hours annealing time.  A total of 32 hours.  This is half the time of adding to the existing slab at each firing.  Multiple 6mm slabs can be fired at the one time if there is space in the kiln, which would reduce the kiln time for the 6mm slabs even further. 

An additional advantage of firing 6mm slabs and combining them, is that bubbles can be squeezed out more easily in the final thick slab fring because of the combined weight of the  slabs.  You could make the individual slabs a little thicker, but that would involve damming each slab.  Not an impossible task of course.  And it would change the calculations, by reducing the number of firings.

Heat Up

Another time saving is to use the second cooling rate from the Bullseye document Annealing Thick Slabs as the first up ramp rate. Take this rate up to a minimum of 540˚C. Although, this is an arbitrary temperature above the strain point to ensure all the glass is above the brittle phase.  It is possible to maintain this initial rate to the bubble squeeze.  But with the slow rises in temperature required for thicker slabs, it is sensible to increase the rate from 540 to bubble squeeze to reduce the firing time.  Once past the bubble squeeze a more rapid rate can be used to the top temperature.  

The heat up times could be about half the minimum cooling times.

A worked example (with certain assumptions) would be:

Firing      thickness       time to top temperature total time.

1            6mm             6.3               

2            9mm             7.1

3            12mm            8.4

4            15mm            10.7

5            18mm            15.9

6            21mm            19.4

7            25mm            25.1

8            28mm            29.1              ca.122 hours

But firing five times for 6mm equals 31.5 hours plus the final firing up of 29.1 hours equals a total of 60.6 hours.  Again about one half the time of progressively building up a base slab to the final thickness.

Savings

This example shows that approximately 90 hours of firing time can be saved by making a series of six millimetre slabs and combining them in a final firing.  There is the additional advantage of reducing the occurrence of bubbles between the layers in the final firing because of the weight of the combined slabs.

Mould Elevation

 

The expansion characteristics of glass and ceramics components

Many people advocate the elevation of moulds.  Mainly for air flow to equalise temperatures above and below the mould.  But also, to prolong the life of the mould.  My observation on these reasons for elevating the mould are that they are not harmful, but not necessary, except for investment moulds.

My experiments have showed insignificant differences in temperatures above and below whether elevated or not.  Since the air temperature under the mould is much the same whether elevated or not, it indicates that elevation of the mould has no significant effect.  But, of course, elevation of the mould does no harm either. 

More important than elevation of the mould, is consideration of the nature of the ceramic mould.  Ceramics have two expansion/contraction temperatures called inversions.  The first is at 226˚C/439˚F, and the second around 570˚C/1058˚F.  The ceramic expands rapidly at these temperatures.  There is a 2.5% increase in volume at 226˚C and a slightly more gradual 1% increase around 570˚C.

This a main reason to use slow ramp rates up to at least 570˚C/1058˚F.  Slower rates ease the ceramic expansion speed and reduces the risk of breaking.  So, slower rates will lengthen the life of ceramic moulds. The cool down for annealing and cooling is slow enough that it presents no risk for the ceramic.

There are occasions when the mould must be elevated, though.  These are when the mould is large, heavy, or damp.  This is to protect the shelf rather than the mould or glass.

 

Heat Up Soaks

Photo credit: Bullseye Glass Co.

It is often advocated that there should be a soak at the strain point to even out the temperature throughout the glass.

My question continues to be why? 

The glass has survived whatever rate has been used up to that point during its brittle phase.  So, it already has every chance of surviving a rapid rate during the plastic phase.

Instead of a soak at the strain point, Bob Leatherbarrow indicates a soak during the brittle phase will be more successful in avoiding heat up breaks.  He has observed that heat up breaks are most likely to happen around 260ºC/500ºF.  Therefore, a soak in that region is most likely to be of use in evenly distributing the heat effectively through the glass rather than at a higher temperature.  He recommends up to a half hour soak there before proceeding at the same rate to the strain point (about 540ºC/1004ºF).  The ramp rate to this heat up soak in the brittle phase should be related to the thickness of the glass and the intended profile.

The thickness to be fired for is determined by the profile.  Rates for full and contour fusing can be as for the thickness before firing.  Rounded tack fuse needs to be fired as though twice as thick, and sharp tack or laminated fuse need to be fired as though 2.5 times.  More information on initial ramp rates to the strain point can be found in Low Temperature Kilnforming available from Bullseye and from Etsy

Thermocouple Placement

Photo credit: Kiln Frog

Sometimes it is difficult to replace a kiln shelf back into the kiln with work on it.  This is normally because the thermocouple sticks out from the wall of the kiln.  Questions have been asked if the thermocouple can be below the shelf to make it easier to place.  Others have asked about reducing the distance into the kiln that the thermocouple projects into the kiln.

These are both bad ideas.

 

Thermocouple Under the Shelf

It is not a good idea to have thermocouple under the shelf because it will then measuring the air temperature under the shelf.  The air temperature under the shelf can be as much as 100ºC/180ºF lower than above the shelf where the work is being fired.  This will cause an overfire on the way up.  Setting the top temperature for 790ºC/1454ºF may give an actual air temperature of up to 890ºC/1634ºF!

 

On the way down at annealing temperature, the air temperature below the shelf is hotter than the air temperature above. It might be annealing at 582º/1080ºF.  This will result in improper annealing at too high a temperature.  The cool will start too early.  The time at the appropriate annealing temperature will be too little.  And the cool finish at too high a temperature.

 

It would be a disaster of a firing.  Don't do it!

 

Reduce the Distance into the Kiln

Another suggestion is to reduce the distance the thermocouple is into the kiln.  This produces inaccurate readings too.  If the the thermocouple is moveable, you can pull it out while inserting the shelf, but it must be re-inserted to the original length before firing, to avoid overfiring.

 

If the thermocouple is not fully inserted, it records a lower temperature than when fully inserted.  I know this from bitter experience.  This results in the pieces being overfired.  But also in inadequate annealing, just as when the thermocouple is under the shelf.

 

 

The placement of the thermocouple is critical to the reading of the air temperatures in the kiln.  The thermocouple should not be moved unless absolutely necessary.  If it is moved, it must be checked to be in the same location as originally placed, because if it is not replaced exactly the temperature readings will be different than previously.

Deep Slumps with Bubbles

 

Photo Credit: Rachel Meadows-Ibrahim

The main causes of the large thin bubble is most probably  too high a temperature combined with a long soak.

Elevation of the Mould

The poster indicated there are eight holes total – four on the sides and four under the glass. This means any air has an exit out from under the glass and from the inside of the mould. So, in this case it does not need to be elevated for exit of air.  In my practice l have never, except in tests, elevated my slumping moulds. I have not had failures. My experiments involved in writing the eBook Low Temperature Kilnforming  showed no significant temperature differences between elevating, or not, below the mould.

Effect of Fast Rates

Slow rates to low temperatures with long soaks avoid sealing the glass to the mould. This means air can move out from under the glass during the slump. 

• Fast rates, and elevated temperatures can restrict air movement from under the slumping glass.  
• Fast rates and high slump temperatures can each cause uprisings because the glass slides down the mould during the soak, and that weight pushes the bottom upwards.

Temperature and Uprisings

This uprising is different from the bubble at the bottom on this piece. It is possible to see the glass bubble is thinner than the surrounding glass. As there were holes for air to escape, it seems the temperature and or speed was great enough to allow the glass to form to the mould at the bottom.  This covered the air holes and allowed the remaining air to push upwards on the glass.  A lower top temperature may have avoided this bubble formation.  Certainly, a combination of a slower rate and a lower temperature would have avoided the formation of the bubble.

Observation

Further, observation during the firing would have caught this bubble formation early enough to skip to the annealing and result in a piece with only a slight uprising, and before it became a bubble. Peeking should start at the beginning of the slumping soak and be repeated at 5 to 10 minute intervals.

Fire polishing Bottles

“I've cut wine bottles horizontally and want to keep the boat shape but round the cut edges. [Will a tack fuse firing] do what I am wanting without changing the shape of the bottle? “

Temperatures

The softening point of bottle glass is around 720˚C. The temperature you have chosen for a fire polish is 730˚C. It will slump to some degree from about 700˚C.  That will not be high enough to fire polish the edges.  Reducing the soak time at 730˚C will reduce the slumping effect a little, but it will not polish or round the edges.

Ground edges

In addition, ground or sawn edges are so rough that fire polishing will not work well at any temperature, because the rough surface promotes devitrification.  To get a good fire polish, the edges should be ground to at least 400 grit, and 600 grit gives a more certain fire polishing result.

Cold Working

Fire polishing is not the most certain way to round and polish edges for a 3D object. Cold working with hand pads or grit is the low cost way to polish the edges.  The grinding will need to go through grits of 200, 400, and then smoothing pads and finally pumice or cerium oxide depending on the shine wanted on the edges.  This can be done by hand or by machine.  Paul Tarlow has an excellent eBook on cold working by hand, and there is some instruction in this blog. 

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.

Visible Devitrification

"Why does devitrification appear on slumped pieces?"

A brief explanation 

Scientific research in developing a glass matrix to support bone grafts gives some information.  This kind of glass matrix requires to be strong.  Development showed that devitrification weakens the matrix.  The crystals in a matrix are not as strong as the amorphous glassy state.  So, devitrification needs to be avoided.

The research to avoid devitrification showed that it begins at about 600˚C/1110˚F.  It only begins to become visible above 700˚C/1300˚F.  The process developed was to introduce a “foaming” agent.  The process fired slowly to 600˚C/1110 ˚F and then quickly to 830˚C/ 1530˚F.  It left a strong open matrix around which bone can grow. Although the research used float glass, it is also a soda lime glass, just as fusing glasses are.  The formation of devitrification begins at the same temperature for fusing glasses as for float.

The result of this medical research shows that devitrification begins on glass before it is visible. Devitrification is cumulative. A little becomes greater with another firing.  This is so even with good cleaning between firings. The new devitrification builds on the previous.  It does this from 600˚C/1110 ˚F.

A subsequent firing can continue this devitrification to the point where it is visible. This can happen, although the temperature at which we can see it after one firing has not been reached.  This continued devitrification at low temperatures can become great enough to be visible at the end of one or multiple slumps.

Credit: Bullseye Glass Co.

What can we do?

Clean all the glass before every firing very well.

·         Avoid mineralised water.

·         Final clean with isopropyl alcohol.

·         Polish dry at each stage with white absorbent paper.

 

Soak longer at lower temperatures.

·         Use longer soaks to achieve the slump.

·         Keep the temperature low.

·         Observe the progress of the firing with quick peeks.

 

Use slower ramp rates.

·         Slower rates enable the heat to permeate the glass.

·         Enables a lower slump temperature.

 

If there is any hint of devitrification after the first firing,

·      use a devitrification spray, or

·      provide a new surface.

• o   remove the surface by abrasion on sandblasting,
• o   cap with clear, or
• o   cover whole surface with a thin layer of clear powder.

·      Fire to contour fuse to give a new smooth surface.

·      Clean very well and proceed to slump.

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.