Refractory fibres

 

There is a lot of imprecise terminology for refractory fibre paper and blanket.  My interpretation:

Shelf paper is a very thin - like cartridge paper - material held together with organic binders, and often containing fibreglass particles.  Thinfire and Papyros are two brand names.

Thinfire. photo credit Warm Glass

Fibre paper is rougher than shelf papers. The fibres are longer and not compressed so much. They seem to be available in 2-6mm thicknesses and are held together with organic or chemical binders.

Refractory fibre paper.  Photo credit Laurel Refractory

Fibre blanket tends to be uncompressed fibre from 12-75mm thick.  It relies on the interconnected fibres rather than binders to keep its thickness.

Refractory fibre blanket. photo credit Amazon

More information in this blog post. 

Lead Testing Kits Evaluation

An example of a lead testing kit from Amazon

There is legitimate concern about lead content of some glass intendended for culinary use.  Surface lead testing kits have become popular and indicate the presence of lead on many glasses.  It seemed to me that some evaluation of home lead test kits was in order.  I looked at some sites for scientific evaluations and some reviews of testing kits and found these results. 

Public Lab, whose mission is “Pursuing environmental justice through community science and open technology”, reports in the paper, “Evaluating Low-cost Lead Screening Products”, by Read Holman that “There are two evaluated [surface lead] test kits, the remaining three for surfaces have not been scientifically evaluated.” The report states that the tests for

“Paint/Surfaces...

• ESCA Tech, Inc. D-Lead Paint Test Kit. This product was "EPA-recognized" in 2010, but for negative results only; the rate of finding false-positives is 16% (Source PDF))
• 3M LeadCheck Swabs. This product was "EPA-Recognized" in 2010, but for negative results only; the rate of finding false-positives is 98% (Source PDF)). This is an extremely high false-positive rate.”

Source

There are seven other scientifically evaluated tests for dust and water, which are not applicable to glass surfaces.

 

The conclusion of a report for the US Dept of Commerce states:

“Currently available spot test kits cannot be used to determine lead-based paint, which is defined as a paint having lead at levels equal to, or greater than, 1mg/cm2 [the allowable level]. This finding was consistent with conclusions from several previously published field studies. As was found in the field studies, the spot test kits in this controlled laboratory study generally gave relatively high percents of false positives at the lead-based paint level of 1 mg/cm². That is, the spot test kits were generally sensitive to lead in paint at much lower levels” (p61)

Source 

The experience of people using these tests (reviews on Amazon) show that almost all surfaces show traces of lead, but at much lower concentrations than the allowable levels. 

A sample review:

“We got a heart attack because what we wanted to test turned positive, we proceeded to then test other stuff as a control, and guess what? All positive.  We got suspicious and started testing random objects that couldn’t possibly contain lead. They also turned positive!”  JSP Lead Test Kit

 

The high levels of false positives (up to 98%) leads me to question their value or accuracy.  Although I am not going to spend money on any of these tests, I suspect the test kits will show lead on clear glass too.

My conclusion is that these tests are not reliable indicators of risky levels of lead presence on the surface of glass artifacts.  Any concern needs a much more reliable test than the currently available surface lead test kits.

 

The Importance of Viscosity in Slumping

 What is viscosity?

The official definition is that it is a measure of the resistance to flow, e.g., honey vs water, or hard vs soft glass.  Honey and hard glass have greater resistance to flow. 

Importance of viscosity

In slumping, large differences in viscosity of the combined glasses will have different rates of deformation across the piece.  There is the possibility of uneven slumps as a result.  The stresses between the different viscosities may cause breaks or splits with rapid temperature rises.  Combining large differences in viscosity requires more caution in ramp rates and in annealing and cooling.  Of course, unusual results can be obtained by manipulating time and temperature.

Effect of temperature

Viscosity is affected more directly by temperature than heat and time.

Credit: Bullseye Glass Company

There are frequent statements about viscosity such as dark glass is less viscous than light, or transparent is less viscous than opalescent.  Also, Bob Leatherbarrow ran some slumping testes showing thick glass slumped less at a given temperature than thin.  Further, Ted Sawyer mentioned to me that some opalescent is less viscous than some transparent glass.   My experience is different, so I wanted to test my assumptions against theirs.

Experiment setup

25mm/1" wide strips were suspended with a span of 20cm/8".  Weights were placed on ends to avoid any slipping.  

Does comparative viscosity vary with temperature?

I fired samples at three temperatures and times

• 600C for 30 minutes
• 650C for 1 minute
• 690 for 1 minute

All at 150C/hr to top temperature.  The short soak time for the higher temperatures were because the glass deformed so quickly.

Results

Bullseye glass. Span of 20cm. Fired at 150C/hr to 600C for 30 minutes

            Code - name - deformation from horizontal

0126 Light Cyan              16mm

0243 Translucent White    20mm

0013 Opaque white         21mm

1101 Clear Tekta             21mm

0100 Black                     24mm

0141 Dark Forrest Green 24mm

1122 Red                       24mm

0161 Robbins egg blue    26mm

0137 French vanilla        27mm

1427 Light amber           27mm

1428 Light violet            29mm

0303 Dusky lilac            32mm

1125 Orange                 32mm

0147 Deep cobalt blue   33mm

0113 White  (.0038)      34mm

0126 Orange                 35mm

1246 Copper blue          37mm

1320 Marigold yellow     40mm

1341 Ruby pink sapphire 40mm  

(special production)

Most opals in this test were more viscous than the transparent glasses.  There are some exceptions such as Dusky lilac, Cobalt blue, Orange.  There were some exceptions too in the transparents: black, red, light amber.

Bullseye glass. Span of 20cm. Fired at 150C/hr to 650C for 1 minute

            Code - name - deformation from horizontal

0100 Black                    26mm

0013 Opaque white        30mm

1122 Red                      30mm

1428 Light violet           30mm

0243 Translucent white  31mm

0141 Dark forest green 31mm

0161 Robins egg blue    31mm

0147 Deep cobalt blue   32mm

0126 Orange opal          32mm

1101 Clear tekta           33mm

1125 Orange                34mm

0137 French vanilla       35mm

0216 Light Cyan            38mm

0303 Dusty lilac            38mm

1341 Ruby pink sapphire 39mm

1437 Light amber          41mm

1320 Marigold yellow     41mm

1246 Copper blue          43mm

0113 White  (.0038)      45mm

Some odd results appeared in this firing.  Black deformed least and white most. But in general, the opal was again more viscous than the transparent.  Exceptions were the red, and light violet in the transparents; and among the opalescents were the light cyan, dusty lilac and white.

Also of note is that the amount of deformation was very similar for the test at 600C for 30 minutes and the one at 650C for only 1 minute.  This re-inforces the concept that time and temperature are often interchangeable, so longer at a low temperature can equal the heat work effects of a shorter soak at a higher temperature.

Bullseye glass. Span of 20cm. Fired at 150C/hr to 690C for 1 minute

            Code - name - deformation from horizontal

0013 Opaque white        35mm

0141 Dark forest green   41mm

0137 French vanilla        44mm

1101 Clear                    49mm

1428 Light violet            52mm

0126 Orange                 53mm

0303 Dusty Lilac            54mm

1437 Light amber          54mm

0113 White   (.0038)     54mm

0243 Translucent white  55mm

1125 Orange                 56mm

1341 Ruby pink sapphire 59mm

1122 Red                      59mm

0161 Robins egg blue     60mm

0147 Deep Cobalt blue   62mm

1320 Marigold yellow     67mm

1246 Copper blue          90mm

The results of the higher temperature in this test showed variations in comparative viscosity.  Some opals (e.g., dark cobalt blue, robins egg blue) were less viscous than most transparents, but some transparents (e.g., light violet and light amber) were more viscous than most opals.

The test shows wide variability in the viscosity of transparent colours at a higher temperature.  It appears that hot and deep colours are the least viscous of the transparent colours in this test.  There are also significant differences in the viscosity of opalescent and transparent glasses of the same colour.  It is apparent that not all glasses have the same rate of viscosity change with the same rate of temperature change.

Summary

This test showed that in general, the opals in the test are stiffer than the transparent from 600C to 690C with some exceptions.  It appears transparent hot colours are less viscous than the light transparent colours.  This is not the same for opalescent colours which seem to have a wider range of viscosity at these temperatures.

The similar deformation of the test glasses at 600C for 30 minutes and at 650C for one minute, shows the possibility of using lower temperatures and longer times to achieve the same effects in slumping as at higher temperatures with shorter soaks.

Viscosity and expansion rate are roughly related at lower temperatures, but both change rapidly above the softening point.  This experiment demonstrates that expansion rates vary within a single fusing compatible range of glass.  Also, glass with significantly different viscosities can be compatible, since this was all Bullseye fusing compatible glass.

It is apparent from this unscientific experiment that when preparing for slumping an important piece that combines different colours and styles, testing for relative viscosity is a good idea to determine if there are widely different viscosities.  This knowledge will enable an accommodation to be made in scheduling.

Tom Sawyer comments on the subject of viscosity:

“Viscosity is not always lower for transparent glasses than for opalescent glasses.  Opalescent glasses will begin to move more at temperatures of 538ºC/1000ºF than will transparent glasses, and even at 677ºC/1250ºF, there are still some opalescent glasses that move more than many transparent glasses.  It is only when we get to fusing temperatures that we begin to see the majority of transparent glasses moving more than the majority of opalescent glasses.  In general, it is correct that darker glasses will move more than lighter glasses – both because of their chemistries and because of their greater propensity to absorb infrared energy.”

More information on the effects of viscosity in kilnforming can be found in the ebook Low Temperature Kilnforming.

Reducing Annealing Time for Float Glass

Credit: Bullseye Glass Co.

Annealing float glass seems take a long time. The annealing point (Tg) is higher than most fusing glasses, although float glass is part of the family of soda lime glass. This group of glasses should be cooled slowly from annealing temperature to 427ºC/800ºF and below to reduce risks of thermal shock.  This makes a greater temperature range over which to  anneal float than fusing glasses, consequently it extends the cooling time and increases energy expenses.

It does not have to be this way.  Annealing of glass takes place over a range.  This range extends below the published annealing point (Tg).  This is the temperature at which equalisation can most quickly take place, but it is not as energy efficient as starting in the lower range.  Annealing points (Tg) vary between manufacturers, but these are some of them:

Pilkington Optiwhite              559ºC/1039ºF

Pilkington Optifloat               548ºC/1019ºF

USA float (typical)                548ºC/1019ºF

Australian float (average)      550ºC/1022ºF

The annealing range extends to a practical 38ºC/68ºF below the Tg temperature.  Annealing at a lower temperature can be as effective at the lower portion of the range as at the Tg.  Using a lower annealing soak temperature reduces the temperature range of the first cooling stage by as much as 38ºC/68ºF, and reduces the cooling time without increasing risks of breaking.  It also creates a denser glass according to scientific research.  Denser glass is arguably a stronger glass.

This means that the annealing of float glass can take place at the following reduced temperatures:

Pilkington Optiwhite              521ºC/971ºF

Pilkington Optifloat               510ºC/900ºF

USA float (typical)                510ºC/900ºF

Australian float (average)      512ºC/954ºF

 

This reduces the first cooling stage for 12mm/0.5” Pilkington Optiwhite from 2 hours 24 minutes to 1 hour 43 minutes.  Forty-one minutes may not seem much but in electricity costs is significant.  Also using the Bullseye concept of a three stage cooling, further savings can be made.  Their research shows the second cooling stage to 371ºC/700ºF can be increased by 1.8 times the first cooling rate, saving further time and energy.  The chart which shows these rates is Annealing Thick Slabs -  Celsius and - Fahrenheit.

More information on annealing is available in the ebook Annealing: Concepts, Principles and Practice

Annealing float glass at the lower part of the annealing range reduces the time and cost of firings.

Grinder Bit Chipping Glass

Credit: Techniglass.com

A new grinder bit chips the glass excessively, especially with a coarse grit. It can also be the result of a bare spot on the bit.  You need distinguish between these states.   Check the surface of the bit.  If there are any small bare spots, the bit needs to be replaced.

 

Credit: WWGrainger

The best thing to do with a new coarse bit is to treat it with a dressing stone.  This is a block of aluminium oxide which can remove high points on the bit, and clean up the spaces between the diamonds on the bit.  It is relatively inexpensive to buy and lasts a long time.  The dressing stone can be a brick, although it is not as efficient because it is much softer.  

If the grinding bit still chips off too much glass from the edge, you need  a finer grit.  It will not take glass off as quickly as the coarse one, but it eliminates or reduces the chipping.  The three common grades are: coarse, standard, and fine.  It is a good idea to maintain a stock of the medium and fine grit grinder bits as replacements for worn ones.

Core drill bits

Credit: JMbestglass.cn

Using core drill bits needs a drill press. It keeps the drill bit steady and avoids breaking the core which plugs the hollow part of the bit.

Oscillating a core diamond drill bit is not the correct procedure. Oscillating the bit creates two undesirable things.

• It breaks off the core that  is being drilled out, plugging the drill bit, and blocking the cooling water being pumped to the drill bit.  This means the bit heats up and loses some of the diamonds. Additionally, it can heat up the glass so much that it breaks. If you are not using a flushing head with your drill, you will need to raise the bit a little from time to time, allowing water to the grinding surface. 
• Starting at an angle or oscillating with a core bit wears out the sides of the drill bit more quickly than necessary. Core drill bits need to be applied directly and vertically. This is why core bits do best in a drill press. It holds the bit in a vertical position without breaking the core being drilled out, or prematurely using the diamonds higher up the bit.

Credit:  Lawson-HIS

There are generic drill presses available for holding Dremel-type craft motors and hand-held drills. They are inexpensive and make the drilling process so much more certain to regulate the pressure. It also makes an easier start without skipping over the glass. They are so inexpensive that a few holes without skipping will pay it.

Credit: Bhole ST1542 Pico Dril

Drill speeds should be varied according to the size of the hole being drilled. This is important with the high speed Dremel-type motors.  Larger holes need a slower speed than smaller ones. The rim speed of a small bit is nearer the rpm of the drill than a larger one, because the larger one travels a greater distance per revolution than a small one. A listing of recommended speeds is given in this blog.

Hollow core diamond bits are of two types:

•     One, where a heating process attaches the diamond, is called sintered in Europe and other countries.
•     The second, where the diamond is bound with resins, is called bonded in Europe.

They seem to have different designations in North America.

Bits of the first type are longer lasting, and more expensive. These can be “sharpened” with an aluminium oxide dressing stick to expose new diamonds and maintain their effectiveness.

Credit: W W Grainger.com

Bits of the second type wear quickly and should not be “sharpened” with a dressing stone. The normal wearing away of the bonding material exposes the new diamonds.  Dressing them wears away the diamonds that could be used in drilling.

Another advantage to core bits, is that a core drill grinds out much less glass from the hole than a solid drill bit, so it takes less time to drill a hole.

One disadvantage, especially on core drills of 5mm and less, is that the core needs frequent cleaning out of the cores that get stuck inside the drill bit. To maintain efficient and effective drilling, the core needs to be poked out from the bit from the base toward the drilling surface.  This applies whether water is being pumped through the core or not.  Without clearing the core, more pressure must be used to continue drilling, resulting in larger break outs as the hole is completed, and more breaks of the complete piece.

Rigidisers - Application and Use

credit: Scarva

 

Material

Rigidisers are colloidal solutions of silica or quartz with a carrier of some form.  It is also available as a powder to mix with water according to the instructions.

Health and Safety 

Silica and quartz (sometimes referred to as flint) in dry powdered form are a serious health risk.  Wear good respiratory protection and long sleeves and gloves against its skin irritant.  Work outside with the powdered form to keep the dust out of the studio. Clean clothing immediately after working with the powdered form of rigidiser.  Wearing gloves is a good idea whenever working with rigidisers, as the wet form is also a strong skin irritant.

Application

Mix up the powdered form as 1 part powder to 4 parts water, by volume.  Do this masked and gloved, and outdoors if possible.  If not, have a HEPA vacuum running next to your work area.  Mix thoroughly and allow to slake for 24 hours.  Then mix very well by hand or with a blender.  Strain the mix to remove any clumps - they can be made into a paste and added to the main solution.

Liberally paint the solution onto the refractory fibre.  Stir prior to use and frequently throughout the application to keep the silica/quartz in suspension.  Depending on permanence, coat one or both sides of the paper/blanket/board.  It is not necessary to soak the fibre completely.  The object is to provide a hard surface.  It does not need to be hard throughout.

Flat Board

It is best to apply rigidiser on both sides of refractory board.  If rigidising both sides, allow one side to air dry before turning over to coat the other side.  By coating both sides, the warping from heating on one side is reduced. 

Slumping forms 

Cover the shape you are taking the mould from with an impervious separator such as Vaseline or thin plastic film.  Prepare the fibre blanket by coating both sides of the fibre with the rigidiser.  It does not need to be completely soaked.  Press the fibre firmly into/onto the shape and especially into any depressions and around any protrusions to be certain of a faithful replication.

Curing  

Allow the refractory fibre to air dry.  Or if needed quickly, you can kiln dry at 90˚C – 110˚C / 194˚F – 258˚F for several hours.  But only if the master mould can withstand the heat.  If not, demould only after the fibre is dry and can hold its shape without the master.  Be sure to remove the master mould from the fibre before proceeding to heat cure.

When air dried, cure in the kiln by firing to 790˚C/1454˚F for 20 minutes.  Before firing, place the dry form on a refractory fibre separator to avoid the silica/quartz sticking to the shelf. A rapid rate straight to the top temperature is acceptable.  After the soak, turn the kiln off, as the rigidised refractory material is not subject to thermal shock.

In Use

Coat the hardened fibre in kiln wash, or cover with shelf paper or refractory fibre paper, to avoid glass sticking to the hardened board.  The bare surface of the rigidised form is now coated in glass fibres and they will stick to the glass unless a separator is applied.

When used as a shelf, it is best to turn the board over after a few dozen firings. This helps counteract the warping tendency that rigidised boards have.

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.