Differential Cooling of Transparent and Opalescent Glass

A statement was made on a Facebook group that transparent glass absorbs more heat than opalescent glass. And it releases more heat during cooling. The poster may have meant that the transparent heats more quickly than the opalescent, and cools more quickly.

Yes, dark transparent glass absorbs heat quicker than most opalescent (marginally), and it releases the heat more quickly (again marginally) than opalescent. The colour and degree of transparency do not absorb any more or less heat, given appropriate rates. They gain the same heat and temperature, although at slightly different rates due to differences in viscosity.

An occasional table

The rate of heating and cooling is important in maintaining an equal rate of absorption of heat. The temperature of both styles can become the same if appropriate lengths of heating, annealing, and cooling are used. The slightly different rates of heat gain can give a difference in viscosity and therefore expansion.  This slight mismatch during rapid ramp rates, might set up stresses great enough to break the glass. This can occur on the quick heat up of glass during the brittle phase (approximately up to 540ºC/1005ºF). In fact, most heat-up breaks occur below 300ºC/540ºF.

The main impact of differential heat gain/loss is during cooling. Annealing of sufficient length eliminates the problem of differential contraction through achieving and maintaining the Delta T = 5C or less (ΔT≤5C). It is during the cooling that the rates of heat loss may have an effect. The marginally quicker heat loss of many transparents over most  opalescent glass exhibits different viscosities and rates of contraction. The stresses created are temporary. But they might be great enough to cause breaks during the cooling. Slow cooling related to the thickness and nature of the glass takes care of the differential contraction rates by maintaining small temperature differentials.

Significance of Differential Heat Gain/Loss

Uneven thicknesses and the tack fusing profile both have much greater effects than the differential cooling rates of transparent and opalescent glass. It may be that strongly contrasting colours (such as purple and white) are also more important factors in heat gain and loss than transparent and opalescent combinations.  Cooling at an appropriate rate to room temperature for these factors will be sufficient to remove any risk of differential contraction between transparent and opalescent glasses.

Effect of Air Space Around Shelves

The Bullseye research on annealing thick slabs indicates that it is important to have a 50mm space between the shelf and the kiln walls. This is to assist even distribution of the air temperature above and below the shelf.

I decided to learn what the temperature differences are between ventilated and unventilated floors of kilns. The recording of the temperatures was conducted using pyrometers on the floor of the kiln and in the air above the kiln shelf. The pyrometer above the shelf was at the height of the kiln’s pyrometer. The recording was done during normal firings of glass. The graph below shows temperature differences during a typical firing.

The blue line indicates the air temperature, the orange line the floor temperature and the grey line the difference in the two over the whole firing. Each horizontal line is 100C

The next graphs show in more detail the differences between having no significant space and another firing with space between shelf and kiln walls.

Horizontal axis legend:

1.  = 300°C
2.  = Softening point
3.  = Top of Bubble Squeeze
4.  = Top temperature
5.  = Start of anneal soak
6.  = start of first cool
7.  = start of second cool
8.  = start of final cool
9.  = 300°C
10.  = 200°C
11.  = 100°C
12.  = 40°C

The general results are that there is a greater difference during the rise in temperature and a reducing difference in floor and air temperature during the anneal cool. However, there are significant differentials at various points during the firings.

Space between the shelf and kiln walls:

• Smaller temperature difference is experienced on the heat up.
• Floor stays hotter than the above shelf air temperature during the anneal soak.
• This difference gradually equalises during the anneal cool

Without space between the shelf and kiln walls:

• Significantly greater difference on heat up is experienced – over 100°C cooler than ventilated floor area.
• Floor temperature is less than air until the final cool.
• During the anneal soak the floor temperature difference becomes larger than at start of anneal. This seems to be the consequence of heat continuing to dissipate through the kiln body, while the air temperature above the shelf is maintained at a constant temperature.
• The difference between the air and floor temperature gradually reduces during the anneal cool as the whole kiln and its contents near the natural cooling rate of the kiln.

 

This appears to indicate that space between the shelf and kiln walls helps to equalise the temperature during the critical anneal soak and first stage of the anneal cool. This will be particularly important when annealing thick slabs.

These tests were done in a kiln of 50cm square. It is likely that the differences would be greater in a large kiln, making it more important to have the air gap between shelf and kiln wall. Smaller kilns and thinner glass seem to be less affected by these differences.

Note that the air temperature and shelf temperature differences in these firings maintain the same character whether the floor has good circulation or not. The shelf temperature lags behind the air temperature throughout the heat up.

The fact is that floor and air temperatures are nearer each other with air space around the shelf. The difference reduces during the bubble squeeze and the top temperature soak. The difference in temperature on cool down is small. During the anneal soak and cool, the shelf tends to be a few degrees hotter than the air temperature.

There was no difference in the amount of stress in the glass in these tests on a small kiln whether there was a gap or not between the shelf and the kiln walls.

Implications for kilns with multiple shelves

Those using multiple shelves in a single firing load should take note of the implications from this. It is important to have significant ventilation between layers to get consistent results from firings.

The ideal would be to have larger than 50mm/2” gap around the upper shelf. Possibly 100mm/4” would be a good starting point. This would allow sufficient heat circulation to compensate a little for the lack of radiant heat from the elements.

If you have a really deep kiln and are using three shelves, the ideal would be to start with a 50mm/2” gap around the bottom shelf. Then a 100mm/4” gap around the middle shelf and finally a 150mm/6” gap around the top shelf. This will assist the heat to circulate to the bottom layer.

 

There are greater differences in temperature between the floor and above shelf air temperature when there is no ventilation space around the shelf. This is especially the case during the anneal soak.

Strain Points

A critical range is the temperature around the annealing point. The upper and lower limits of this range are known as the softening and strain points. The higher one is the point at which glass begins to bend.  It is also the highest temperature at which annealing can begin. The lower one is the lowest point at which annealing can be done. Soaking at any lower temperature will not anneal the glass at all. This temperature range is a little arbitrary, but it is generally considered to be 55C above and below the annealing point. The ideal point to anneal is thought to be at the annealing temperature, as annealing occurs most rapidly at this temperature.

Annealing Range

However, glass kiln pyrometers are not accurate in recording the temperature within the glass, only the air temperature within the kiln. The glass on the way down in temperature is hotter than the recorded kiln atmosphere temperature. A soak within the annealing range is required to ensure the glass temperature is equalised. If you do a soak at 515°C for example, the glass is actually hotter, and is cooling and equalising throughout to 515°C during the soak. The slow cool to below the lower strain point constitutes the annealing, the soak at the annealing point is to ensure that the glass is at the same temperature throughout, before  the annealing cool begins.

Strain Point and Below

No further annealing will take place below the strain point. If you do not anneal properly, the glass will break either in the kiln or later no matter how carefully you cool the glass after annealing.

It is still possible to give the glass a thermal shock at temperatures below the lower strain point, so care needs to be taken.  The cool below the anneal soak needs to be at a slow controlled rate that is related to the length of the required anneal soak. Too great a differential in contraction rates within the glass can cause what are most often referred to as thermal shock.  The control of the cooling rate reduces the chance of these breaks.

Softening Point

The glass is brittle below the softening point temperature, although it is less and less likely to be subject to thermal shock as it nears the softening point.  It is after the softening point on the increase in temperature that you can advance the temperature rapidly without breaking the glass.  So, if you have a glass that gives its annealing temperature as 515C, you can safely advance the temperature quickly after 570C (being 55C above the annealing point).

Characteristics of Some Glasses

This information has been taken from various sources. Some manufacturers may change the composition of their glasses or the published information about them from time to time. Therefore, this information can only be used as a guide. If the information about strain, annealing, and softening points is important, contact the manufacturer for the most accurate information.

The temperature information is given in Celsius.

Strain point – the temperature below which no annealing can be done.

Annealing point – the temperature at which the equalisation soak should be done before the annealing cool.

Softening point – the temperature at which slumping can most quickly occur.

Armstrong – Now made by Kokomo

Typical Borosilicate – nominal CoE 32

Strain point – 510 - 535C / 951 - 996F

Annealing point – ca. 560C/1041F
Softening point - ca. 820C/1509F

Blackwood OZ Lead – nominal CoE 92

Annealing point - 440C/825F

Blenko – nominal CoE 110

Annealing point – 495C/924F

Bullseye – nominal CoE 90

Transparents

Strain point - 493C/920F

Annealing point - (532C)  Note that Bullseye has changed this to 482C/900F for thick items

Softening point - 677C/1252F

Opalescents

Strain point - 463C/866F

Annealing point – (501C)  Note that Bullseye has changed this to 482C900F for thick items

Softening point - 688C/1272F

Gold Bearing

Strain point - 438C/821F

Annealing point - (472)   Note that Bullseye has changed this to 482C/900F for thick items

Softening point - 638C/1182F

Chicago – nominal CoE 92

Desag  Note that this glass is no longer produced

Artista – nominal CoE 94

Strain point – 480 - 510C / 897 - 951F

Annealing point – 515 - 535C / 960 - 996F

Softening point – 705 – 735C / 1302 - 1356F

Fusing range – 805 – 835C / 1482 - 1537

Float Glass (Pilkington UK)

Optiwhite

Strain point – 525 - 530C / 978 - 987F

Annealing point – 559C/1039F

Softening point – 725C/1338F

Optifloat

Strain point – 525 - 530C / 978 - 987F

Annealing point – 548C/1019F

Softening point – 725C/1338F

Float Glass (typical for USA) nominal CoE 83

Strain point - 511C/953F

Annealing point - 548C/1019F

Softening point – 715C/1320F

Float Glass (typical for Australia) nominal CoE 84

Strain point - 505-525C / 942 - 978F 

Annealing point – 540 -560C / 1005 - 1041F

HiGlass “GIN” range – nominal CoE 90

Annealing point - 535C/996F

Gaffer colour rod – nominal CoE 88

Gaffer NZ Lead – nominal CoE 92

Annealing point - 440C/825F

HiGlass

Annealing point - 495C/924F

Kokomo – nominal CoE 92 - 94

Cathedrals

Strain point - 467C/873F

Annealing point - 507C/946F

Softening point - ca. 565C/ca.1050F

Opal Dense

Strain point - 445C/834F

Annealing point - 477C/891F

Softening point – ca. 565C/1050F

Opal Medium

Strain point - 455C/834F

Annealing point - 490C/915F

Softening point – ca.565C/1050F

Opal Medium Light

Strain point - 461C/863F

Annealing point - 499C/931F

Softening point – ca.565C/1050F

Opal Light

Strain point - 464C868F

Annealing point - 502C/937F

Softening point – ca.565C/1050F

Kugler – nominal CoE

Annealing point - 470C/879F

Typical lead glass – nominal CoE 91

Lenox Lead – nominal CoE 94

Annealing point – 440C/825F

Merry Go Round – nominal CoE 92

Moretti/Effetre – nominal CoE 104

Strain Point: 448C/839F

Annealing Range: 493 – 498C / 920 - 929F

Softening Point: 565C/1050F

Pemco Pb83 – nominal CoE 108

Annealing point – 415C/780F

Schott Borosilicate (8330) nominal CoE 32

Annealing point - 530C/987F

Schott “F2” Lead – nominal CoE 92

Annealing point - 440C/825F

Schott “H” & “R6” rods - nominal CoE 90

Annealing point – 530C/987F

Schott “W” colour rod – nominal CoE 98

St Just

MNA

Strain point - ca.450C/843F

Annealing point – ca. 532C/ca. 991F

Spectrum

System 96 – nominal CoE 96

Transparents

Strain point – 476C  +/- 6C  /  890F +/- 11F

Annealing point – 513 +/- 6C  /  956C +/- 11F

Softening point – 680 +/- 6C  /  1257F +/- 11F

Opalescents

Annealing point – 505 -515C  /  942 - 960F

Spruce Pine 87 – nominal CoE 96

Annealing point – 480C/897F

Uroboros system 96 – nominal CoE 96

Transparents

Strain point - 481C/899F

Annealing point - 517C/964F

Opalescents
Strain point - 457C/855F
Annealing point - 501C/935F

Uroboros - nominal CoE 90

Transparents

Strain point - 488C/911F

Annealing point - 525C/978F

Opalescents

Strain point - 468C/875F

Annealing point - 512C/955C

Wasser - nominal CoE 89

Annealing point – 490C/915F

Wissmach
Wissmach 90
Annealing point - 483C/900F
Softening point - 688C/1272F
Full Fuse - 777+

Wissmach 96
Annealing point - 483C/900F

Softening point - 688C/1272F

Full Fuse - 777+ / 1432+

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Gravity

One of the fundamental elements in kiln forming is gravity. When glass is hot it moves according to the effects of gravity and you have to remember that gravity has a big effect on all your firings.

The effects mainly cause:

• Uneven thickness on shelves that are not level.
• Uneven slumps into moulds which are not level or the glass is not levelled.
• Uneven forming due to varying viscosities. Gravity acts on the softest parts of the glass first.
• Faster or slower forming due to span width. With greater span, gravity pulls the glass into the mould more quickly than with a small span.
• Gravity acts on things of greater thickness more quickly than those of lighter weight. So a thick piece will form more quickly than the same sized thin piece.
• Surface tension (affected by viscosity and heat) is affected by gravity also. The glass will attempt to draw up or spread out to about 7 mm if there is enough heat, time, and low viscosity.
• The effect of gravity causes upper pieces to thin lower ones, as it presses down while the glass is plastic. This has the effect of making the colour of the lower piece less strong.

More information on each of these effects can be found throughout this blog.

Heat Work

“Heat work” is a term applied to help understand how the glass reacts to various ways of applying of heat to the glass. In its simple form, it is the amount of heat the glass has absorbed during the kiln forming heat up process.

There is an relationship between how heat is applied and the temperature required to achieve the wanted result.  Heat can be put into the glass quickly, but to achieve the desired result, it will need a relatively higher temperature. If you put the heat into the glass more slowly, it will require a relatively lower temperature.

For example, you may be able to achieve your desired result at 814C with a 400C/hr rise and 10min soak. But you may also be able to achieve the same result by using 790C with a 250C/hr rise and 10min soak. The same amount of heat has gone into the glass, as evidenced by the same result, but with different kinds of schedules. This can be important with thick glass, or with slumps where you want the minimum of mould marks. Of course, you can also achieve the same results with the fast rise with a longer soak at the lower temperature, e.g. a 400C/hr to 790C with a 30 min soak.

In short, this means that heat work is a combination of time and temperature.  The same effect can be achieved in two ways: 
- fast rates of advance and high temperatures
- slow rates of advance and low temperatures.

You obtain greater control over the processes when firing at slower rates with lower temperatures.  There is less marking of the back of the piece.  There is less sticking of the separators to the back and so less cleanup.  There is less needling with the lower temperature.  

The adage “slow and low” comes from this concept of heat work. The best results come from lower temperature processing, rather than fast processing of the kiln forming.

Effect of Heat on Sandblasted textures

This is based on Graham Stone’s work with float glass. The temperatures are applicable to float glass, and so need to be adjusted for other glasses, but illustrate the principle of how heating temperatures affect the glass.
Temperatures in degrees Celsius.

650 Blasted surface softened, evened, less "brutal".
690 Blasting still opaque but less "white"
700 Blasting becoming too sheeny but still okay for certain effects.
740 Blasting now subtle and glossy

Based on Firing Schedules for Glass; the Kiln Companion, by Graham Stone, Melbourne, 2000, ISBN 0-646-39733-8, p24

Approximate Temperature Characteristics of Various Glasses

Various glasses have different temperature characteristics. This listing is an attempt to indicate the differences between a variety of popular glasses used in kiln forming. They are not necessarily exact, but do give an indication of differences.

Bullseye Transparents
Full fusing 832C
Tack fusing 777C
Softening 677C
Annealing 532C
Strain point 493C

Bullseye Opalescents
Full fusing 843C
Tack fusing 788C
Softening 688C
Annealing 502C
Strain point 463C

Bullseye Gold Bearing Glasses
Full fusing 788C
Tack fusing 732C
Softening 632C
Annealing 472C
Strain point 438C

Desag GNA
Full fusing 857C
Tack fusing 802C
Softening 718C
Annealing 530C
Strain point 454C

Float Glass
Full fusing 835C
Tack fusing 760C
Softening 720C
Annealing 530C
Strain point 454C

Oceanside
Full fusing 788C
Tack fusing 718C
Softening 677C
Annealing 510C
Strain point 371C

Wasser
Full fusing 816C
Tack fusing 760C
Softening 670C
Annealing 510C
Strain point 343C

Wissmach 90
full fusing  777C
Tack fusing
Softening  688C
Annealing  510C
Strain point

Wissmach 96
Full fusing  777C
Tack fusing
Softening  688C
Annealing  510C
Strain point

Youghiogheny 96
Full fusing  773C
Tack fusing  725C
Softening  662C
Annealing  510C
Strain point

Viscosity Changes with Temperature

This image is taken from Pate de Verre and Kiln Casting of Glass, by Jim Kervin and Dan Fenton, Glass Wear Studios, 2002, p.27.

It shows in graphic form how the viscosity of glass decreases with increases in temperature. The temperatures are given in Fahrenheit.  

The coefficient of expansion also changes with temperature. 

This graph is also from Kervin and Fenton

 It is these two forces of viscosity and expansion that must be balanced around the annealing point to give a stable and compatible range of fusing glass.

Slow and Low

Low and Slow Approach to Kilnforming

We are often impatient in firing our pieces and fire much more quickly than we need. After all, our computerised controllers will look after the firing overnight. So there is no need to hurry more than that.

The concept of heat work is essential to understanding why the slow and low method of firing works. Glass is a poor conductor of heat which leads to many of our problems with quick firings. The main one is stressing the glass so much by the temperature differential between the top and the bottom that the glass breaks. We need to get heat into the whole mass of the glass as evenly and with as smooth a temperature gradient as possible. If we can do that, the kiln forming processes work much better. If you add the heat to the glass quickly, you need to go to a higher temperature to achieve the desired result than if you add the heat more slowly to allow the heat to permeate the whole thickness of the piece.

Graphs of the difference (blue line) between upper and lower surfaces of glass of different thicknesses against cooling time

However, this slower heating means that the glass at the bottom has absorbed the required heat at a lower temperature than in a fast heat. This in turn means that you do not need to go to such a high heat. This has a significant advantage in forming the glass, as the lower temperature required to achieve the shape means that the bottom of the glass is less marked. The glass will have less chance of stress at the annealing stage of the kiln forming process as it will be of a more equal temperature even before the temperature equalisation process begins at the annealing soak temperature.

Applying the principles of low and slow means:

• heat is added evenly to the whole thickness of the piece
• there is a reduction in risk of thermal shock
• the glass will achieve the desired effect at a reduced temperature

The alternative - quick ramps with soaks – leads to a range of difficulties:

• The introduction of heat differentials within the glass. Bullseye research shows that on cooling, a heat difference of greater than 5ºC between the internal and external parts of glass lead to stresses that cannot be resolved without re-heating to above the annealing point with a significant soak to once again equalise the heat throughout the piece.
• It does not save much if any time, As the glass reacts better to a steady introduction of heat. Merely slowing the rate to occupy the same amount of time as the ramp and soak together occupy, will lead to fewer problems.
• It can soften some parts more quickly than others, e.g., edges soften and stick trapping air.
• Quick heating, with “catch up” soaks, of a piece with different types and colours of glass is more likely to cause problems of shock, bubbles, and uneven forming.
• Pieces with uneven thicknesses, such as those intended for tack fusing, will have significant differences in temperature at the bottom.
• Rapid heating with soaks during slumping and draping processes can cause uneven slumps through colour or thickness differences, or even a tear in the bottom because the top is so much more plastic than the bottom.

However there are occasions where soaks during the initial advance in heat are useful:

• for really thick glass,
• For multiple - 3 or more - layers of glass,
• for glass on difficult moulds,
• for glass supported at a single internal point with other glass free from contact with mould as on many drapes.

Of course, if you are doing small or jewellery scale work, then you can ignore these principles as the heat is gained relatively easily. It is only when you increase the scale that these principles will have an obvious effect.

Slow, gradual input of heat to glass leads to the ability to fire at lower temperatures to achieve the desired results, with less marking and less risk of breaking.

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

Equalising Effects on Both Sides of the Glass in the Same Firing

The desire is to have the same degree of fusing on both sides of the glass.  An example is where a person wants to have their colourline paints equally matured on both sides of the glass in one firing.  This is difficult and requires a different strategy than normal fusing.

Background

A bit of background first. Glass is a very good insulator. This means that heat travels slowly through the glass. Its practical effect is that we have wavy lines on the top and very crisp lines on the bottom.  This results from the temperature differential between the two surfaces.  This can be many degrees different during the plastic phase of the glass.  It is dependent on how fast the temperature rise is.  The faster the rise in temperature, the greater the difference as the glass transmits the heat from top to bottom so slowly.  The problem is how to keep the temperature differential as small as possible.

Heat Work

The concept of heat work relates to the way heat is put into the glass.  It can be done quickly to a high temperature, or slowly to a low temperature and still get the same effect.  This shows glass reacts to the combination of temperature and time. Putting heat into the glass slowly allows lower temperatures to be used to achieve the desired effect, than fast rises in temperature.

The insulating properties of glass means that the heat work needs to be applied slowly to achieve similar temperatures on both sides of the glass.  The thicker the glass the longer it will take to temperature equalisation.

The mass of materials also needs to be considered.  The glass will normally be on a ceramic shelf of 15mm to 19mm.  This mass also needs to heat up to the temperature of the top of the glass.  Until it does, it will draw heat from the glass.  This also points to the need for slow heat input.

The question that prompted this note was how to get glass strainers paints to have the same degree of maturation on both sides at the same time.  The maturation temperature of Reusche tracing paints is around 650°C.  If you use a normal rate of advance – say, 200°C – the bottom of the glass will be considerably cooler than the top.  This is both because of the insulating properties of the glass and the mass of the shelf.

Methods to achieve the effect.

Some methods are worthy of consideration separately or in combination.

Use refractory fibre board as shelf.  This dramatically reduces the mass of the shelf to be heated up.  This kind of shelf requires more care to avoid damage than a ceramic shelf.  It would be possible to place smaller fibre shelves on top of the standard ceramic shelf rather than having one large fibre board shelf.  This will not be so efficient an insulating mass as fibre board on its own.  Also, it will not be sufficient on its own to obtain equal temperatures on both sides of the glass.

Use 3-6mm refractory fibre paper between shelf and glass.  This again reduces the heat sink effect of the ceramic shelf, but not as much as a fibre shelf on its own.  Again, the fibre paper on its own is not enough. The scheduling is important.

Use very slow rates of advance.  A slow rate of advance in temperature is important to achieving equal temperatures throughout the glass.  Even using 3mm glass, the rate of advance might need to be as slow as 50°C per hour.  The corollary of this is that you will not need to use as high a temperature to achieve the effect.  Heat work means that it is not an absolute temperature that will achieve the effect.  The slower you put the heat into the glass the lower temperature required.  The understanding of this relationship will require experimentation to establish the relationship to the rate of advance and the top temperature required.  For example, a satin polish of a sandblasted surface can occur at 650°C, if held there for 90 minutes.

In this case, a 50°C rate of advance will probably not require more than 600°C – and probably less - to achieve the shiny surface normally achieved at 660°C with a 200°C rate of advance.  At 50°C per hour, it will take 12 hours to reach 600°C, although a little more than 3.25 hours at an advance of 200°C to reach 660°C.  The input of heat acts upon the glass throughout the process, making lower working temperatures possible.  The reduction in temperature required is not directly related to the reduction in the rate of advance.  You will have to observe during the experimental phase of this kind of process.

If it was desired to fire enamels that mature at 520°C to 550°C you could put the sheets in vertical racks to allow the heat to get to both sides equally as Jeff Zimmer does.  But this will only work for very low temperatures and for quick firings, otherwise the glass will begin to bend.

There are limits to this strategy of getting upper and lower surfaces to the same temperature, both in terms of physics and practicality.  There are temperatures below which no amount of slow heat input will have a practical effect, for example,  due to the brittle nature of the glass.  Even where it is possible, it can take too long to be practical.  For example, I can bend float glass at 590°C in 20 minutes into a 1/3 cylinder.  I could also bend it at 550°C (just 10°C above the annealing point), but it would take more than 10 hours – not practical.

Devitrification Temperature Range

Devitrification is the beginning of crystallisation of the surface of the glass. It can look like a dirty film over the whole piece or dirty patches. At its worst, the corners begin to turn up and become “wrinkly”.

This piece shows both mild devitrification and more severe wrinkling on the right side.

This occurs in the range 730° – 760°C. This means that you need to cool the project quickly as possible from the working (or top) temperature to the annealing point. There is evidence to show that dwelling for a long time in this range on the way up to top temperature can promote devitrification too.

The lower graph line shows the  temperature relationships between annealing (glass transition), devitrification and blowing temperatures.

For more information see this post.

Firing Bullseye and Oceanside Together

Is it possible to fire Oceanside (formerly Spectrum) and Bullseye at the same time?

Yes, it is possible to fire pieces made of Oceanside and pieces made of Bullseye in the same firing – as long as the glass is not mixed in one piece.

There will be differences in profile as the temperatures for Spectrum are a little less than for Bullseye at all stages.  A rounded tack for Spectrum will be a much sharper edged tack for the Bullseye, etc.  If you can accommodate those differences you can continue to fire.

It is a bit easier on slumping operations as you can use the lower slumping temperature for Spectrum and extend the soak for the Bullseye glass.  Or, choose a mould for the Bullseye that requires less time than the Spectrum, so they complete the slump at the same time.

The annealing points are different, of course.  But not by much – Spectrum is 510°C and Bullseye 516°C (for any but thick pieces).  These are not far away from each other.

There are two main approaches to annealing different glass in the same firing.

One is to use a shotgun approach.  This means that you choose your upper anneal soak – in this case 516°C – and hold the temperature for the required amount of time.  Then proceed more slowly than usual – say 50°C /hour rather than 80C/hour – until about 55°C below the lower anneal point.  Then proceed to the rest of the cooling.

The other approach is to anneal soak at both annealing points before proceeding to the anneal cool.  This approach is probably best with thicker than 6mm pieces than the shotgun method.  It is also required if you use the Bullseye lower annealing point of 482C.  You would anneal at 510°C and again at 482°C and soak at each point for the required time for thickness.  This doubles the annealing time, thus reducing the advantage of one over two firings.

There is a third approach for pieces less than 9mm that will eliminate the double anneal soak.  Choose a single annealing temperature.  The two annealing points for Bullseye and Spectrum are so close (510°C and 516°C) that you could chose a mid-point between them (say 513°C) and soak there before proceeding to the anneal cool.  

It might be even better to choose a temperature midway between 510°C and 482°C (say 499°C) and soak both glasses for a longer period to ensure the temperature is equalised before proceeding to a slow rate of anneal cool.  This will be especially applicable for tack fused pieces, which require more care than full fused pieces.  Remember that you should be soaking at the temperature equalisation hold for at least twice the thickness of the thickest part of the piece.  Then reduce the temperature at the rate recommended for the thickness indicated.  Look at the Bullseye chart for annealing thick slabs for the rates. 

The reason that you can anneal at different temperatures is that annealing occurs over a range of temperature.   The annealing point is the temperature at which annealing can most quickly occur.  There are several of physical changes that are affected by temperature and rates of cooling. 

If you cool too quickly after the anneal soak, you will induce stress and probable breakage.  The cooling after the anneal soak is an essential part of the whole annealing process.  Annealing at a lower temperature requires more certainty that the glass is all equal in temperature.  This means a longer anneal (or temperature equalisation) soak is required.  It is also a good bet to slow the anneal cool to be less than you would use for a single glass.

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