Re-firing

A frequently asked question is “how many times can I re-fire my piece?”

This is difficult to answer as it relates to the kind of glass and the firing conditions.

Kind of glass

Float glass is prone to devitrification. This often begins to appear on the second firing. Some times it may be possible to get a second firing without it showing. Sandblasting the surface after getting devitrification will enable another firing at least.

Art glass is so variable that each piece needs to be tested.

Fusing glasses are formulated for at least two firings, and experience shows may be fired many of times. The number will depend on the colours and whether they are opalescent. Transparent colours on the cool side of the spectrum seem to accept more firings than the hot colours. Both of these accept more firings than opalescent glasses do.

Firing conditions

Temperature

The higher the temperature pieces are fired at, the fewer re-firings are possible. So if multiple firings are planned, you should do each firing at the lowest possible temperature to get your result. This may mean that you have relatively long soaks for each firing. The final firing can be the one where the temperature is taken to the highest point.

Annealing

You do have to be careful about the annealing of pieces which have been fired multiple times. A number of people recommend longer annealing soaks. However, I find that the standard anneal soak for the thickness is enough. What is required is cooling rates directly related to the anneal soak.  This is a three-stage cooling as described in the Bullseye chart Annealing Thick Slabs.  The slump firing can be annealed at  the standard. 

Slumping

In general, slumping is at a low enough temperature to avoid any creation of additional stress through glass changes at its plastic temperatures.  But any time you heat the glass to a temperature above the annealing point, you must anneal again at least as slowly as in the previous firing. Any thing faster puts the piece at risk of inadequate annealing.  Of course, having put all this work and kiln time into the piece, the safest is to use the cooling rate as for a piece one layer thicker.  My research has shown that this gives the least evidence of stress.

Testing

Testing for stress after each firing will be necessary to determine if there is an increase in the stress within the piece. In the early stages of multiple firings, you can slow the annealing and if that shows reduced stress, it will determine your previous annealing schedule was inadequate. When reducing the rate of annealing does not reduce the stress, it is time to stop firing this piece at fusing temperatures.

Revised 6 May 2023

Effect of AFAP Rates

 

 

This graph illustrates the effect of a rapid increase (500C/hr) in temperature on the glass.  The blue line represents the air temperature measured in the kiln.  The orange line represents the temperature between the glass and the shelf.  At an air temperature of 815°C, the temperature of the glass at its bottom is around 750°C.  This is a large difference, even though the glass is in the plastic range.  It means that the potential for stress induced by the firing rate is large.  The graph shows the temperature difference evens out during the annealing soak.

 The fast rise in temperature at the initial part of the firing where the glass is still brittle risks breakage.  The difference in temperature between the top and bottom of a 6mm piece of glass is shown to be 100°C plus throughout this initial phase up to 500°C.  Most breaks due to thermal shock occur before 300°C. This large temperature difference that occurs with rapid rates of advance risks breakage early in the firing.

 As an example, I took a piece out at 68°C to put another in.  During the time the kiln was open, the air temperature dropped to 21°C.  I filled the kiln and closed the lid and idly watched the temperature climb before switching the kiln on for another firing.  It took a bit more than two minutes for the thermocouple to reach 54°C with the eventual stable temperature being 58°C.  I had not been aware how long it takes the thermocouple to react to the change in temperature.  Yes, it takes a little time for the air temperature in the kiln to equalise with the mass of the kiln, but not two minutes.

 With a two-minute delay the recorded temperature can be significantly behind the actual air temperature.  For example, a rate of 500°C per hour is equal to 8.3°C (15°F) per minute or 16.6°C (30°F) overshoot of the programmed temperature. Even at 300°C it is a 10°C (18°F) overshoot.  This effect, added to the way the controller samples the temperatures, means the actual overshoot can be significant for the resulting glass appearance.

 This is just another small element in why moderate ramp rates can be helpful in providing consistent results for the glass.

 More importantly at top temperature, the surface will be fully formed while the bottom is only at a tack fuse temperature. This does have implications for the strength of the piece.  There will be an only tack fused structure through much of the piece, but a full fused structure at the surface.  The potential for breaking in further kilnforming or during use is high.

 In addition to the effects on the glass, there will be effects on the operation of the controller.  Controllers operate by comparing the instructions on firing rate with the air temperature recorded by the pyrometer.  In doing this the variances become smaller with time.  An AFAP firing does not give a lot of time for the controller to “learn” the firing curve.  So, the controller tends to overshoot the top temperature by some (variable) degree.  This makes it difficult to precisely control the outcome of the firing.

 There is some concern that the structure of the kiln will be affected by AFAP firings. This is a small risk.  The expansion and contraction of the kiln materials will occur whether quickly or more slowly.  It is not a major concern.  It is a concern for the glass, though.

 AFAP firings have potentially harmful effects on the structure of the fired glass leading to thermal shock and fragile completed pieces.

Making Test Tiles

Creating samples or tests provides both references on firing profiles and knowledge of the characteristics of the kiln. 

General samples

 Sample tiles are normally a series of tiles with the same lay up but fired at different temperatures.  These are likely to be intervals of temperature from a sharp tack to a full fuse.  A suitable interval might be 10°C as it is easy to interpolate between these for a slightly different profile than the tiles show. This is the basic arrangement.

 You can make this more informative by including tiles in the same basic lay up but with hot and cool colours, opalescent and transparent, black and white, strikers, etc.  The addition of these will give a richer bank of information.

 Of course, these tiles must be labelled with glass types and code numbers and the temperature used.  This is not all the information required though.

 Many sample tile sets do not include the firing rate.  The heat work required to attain a specific profile is dependent on time, temperature and hold.  These are the time to get to the working temperature, the temperature, and the soak time.  If you do not record the ramp rate used, you will have incomplete information.  It is not that you have to record the entire schedule.  But the rates and any soaks on the way to the top temperature need to be recorded. This means you can take account of any slower rate of heating, any additional holds on the way up, and the length of the soak at top temperature.  Then when contemplating something more complicated than the conditions under which the tiles were made you have better information.

 It is a good idea to maintain a photographic record of the sample tiles to avoid storage problems.  These can be made from the individual tiles and photographed from several angles. 

 Another way of keeping records - without making tiles for each temperature - is to photograph the tiles through peep holes as the set temperatures are achieved.  This means the tiles need to be placed in the kiln so they can be seen from the peep hole. You will only have a physical sample for the top temperature. The other profiles will have a photographic record.  The firing conditions for these need to be recorded just as for the series of physical tiles.

 This photographic record may not be suitable for your way of working and so require making the sets of multiple tiles.  Both these methods provide a generalised record of heat work to achieve given profiles.  Note that you will need to prepare sample tiles for each kiln, as each has different characteristics. 

Specific samples

 However, there will be cases where the general conditions exemplified by the reference tiles are to be exceeded.  In this case you will need to make a sample specific to the piece you are planning.  This can be a general representation of the piece, or a scaled mock-up. 

 The general test tile may be small scale or relatively large.  It will contain only the components in terms of height and shape that will be in the planned, but more complicated piece.

 A more rigorous method is to make a full scale – or nearly so – mock-up of the piece. This is usually done in clear. Fire it to the proposed schedule to determine the exact effect.

  

Sample making gives confidence in preparing work for the kiln and scheduling to get the desired profile.

Making Circles from Fused Squares

Dennis Brady has done a lot of work on predicting the size of circles resulting from stacking squares of glass and taking them to full fuse for enough time to allow flattening of the stacks.  This may be up to half an hour at 815°C for Bullseye.  Some observation will be required.

Stacks of 12mm/ 0.5" squares arranged at 45° to each other and taken to a full fuse:

• 1 layer should produce a 10mm/ 0.375" circle
• 2 layers should produce a 12mm/0.5" circle
• 3 layers should produce a 16mm/0.6" circle
• 4 layers should produce a 18mm/0.7" circle

 Stacks of 19mm/0.75" squares arranged at 45° to each other and taken to a full fuse:

• 1 and 2 layers will not fully round
• 3 layers should produce a 28mm/1.1" circle

 Stacks of 25mm/1" squares arranged at 45° to each other and taken to a full fuse:

• 4 layers should produce a 40mm/1.6" circle
• 5 layers should produce a 45mm/1.75" circle
• 6 layers should produce a 50mm/2" circle

 Stacks of 32mm/1.26" squares arranged at 45° to each other and taken to a full fuse:

• 4 layers should produce a 48mm/1.9" circle
• 5 layers should produce a 52mm/2" circle
• 6 layers should produce a 58mm/2.3" circle

 Stacks of 37mm/1.5" squares arranged at 45° to each other and taken to a full fuse:

• 4 layers should produce a 40mm/1.6" circle
• 5 layers should produce a 45mm/1.8" circle
• 6 layers should produce a 50mm/2" circle

 Stacks of 50mm/2" squares arranged at 45° to each other and taken to a full fuse

• 4 layers should produce a 75mm/3" circle
• 5 layers should produce a 85mm/3.3" circle
• 6 layers should produce a 95mm/3.75" circle
• 7 layers should produce a 102mm/4" circle
• 8 layers should produce a 105mm/4.125" circle

 Based on work done by Dennis Brady

I had a few queries about this regular progression and wondered if it applied to opalescent as well as transparent glass. I set up a few tests in my kiln. I fired them at 400C to 815C for 10 minutes.

I got the following results. 

You can see that the opalescents require more heat work than the transparent. If you are making circles with both transparent and opalescent you will need more time at the top temperature - perhaps 30 to 45 minutes.  This results from the greater viscosity of the opalescent colours.

I also tried making ovals from rectangular pieces oriented at about 25 degrees to each other. You can see they were not successful with a 10 minute soak.  

Hake brushes

Hake (ha-kay) brushes are made from goat's hair. Their advantage over other brushes for applying kiln wash is that they hold a lot of liquid. Proper ones made from joined bamboo work better than the ones with flat handles.

Traditional Japanese hake brush

People often note that these brushes tend to shed hairs. The solution to stray hairs (given to me in a Bullseye workshop) is to invert the new brush and apply super glue at the point where the hairs emerge from the handle.  This holds the hairs in place. It will work on flat handles too.

Inexpensive goat's hair brushes of the hake style.
As can be seen by comparison, there are fewer hairs in these.

Kiln wash application with a brush

Kiln wash is applied thinly in a 1:5 powder to water mix to shelves and moulds.  The object is to get a complete coverage with a smooth surface. 

To ensure full coverage painting four coats is sufficient for excellent coverage.  The kiln was should be applied in four directions – horizontal, vertical, and each diagonal.  This ensures any gaps in one coat will be covered by the others. A broad brush that holds a lot of liquid provides good coverage.  A hake brush is ideal.  The brush should be held almost vertical with the ends of the bristles only touching the surface.

A traditional Japanese hake brush

There is no need to dry each coat before applying the next. It is not like painting your wall. All coats can be applied one directly after the other. No drying between coats is required.  In fact, earlier dried coats tend to make the application clumpy and streaky.

Some people advocate a fifth coat.  I don’t know what the fifth coat is for. What direction other than the four cardinal ones can there be?  It maybe it is insurance that the surface is coated evenly.  This can be checked visually.  The kiln washes used for glass are routinely coloured.  If the shelf shows unevenly through the kiln wash, a little more needs to be brushed onto the more thinly coated area.

It is possible to smooth the kiln washed surface once the kiln wash has a dusty surface – it does not have to be completely dry – you can put a piece of paper between the shelf or mould and your hand.  Gently rub the surface to get a really smooth finish to your kiln washed shelf.

 

More information:

https://glasstips.blogspot.com/2009/08/applying-kiln-wash.html

https://glasstips.blogspot.com/2009/08/smooth-kiln-wash-on-shelves.html

Annealing Range

NOTE: completely revised 31 December 2021

After Bullseye published annealing tables for thick slabs, some people feel they need to use the lower part of the annealing range for all their glass. To determine whether or when to use these tables needs some understanding of the annealing range.

Range

The annealing range of a glass is approximately 40ºC/72ºF on either side of the annealing point, but for practical kiln forming purposes it is normally taken as 33ºC/60ºF. The annealing point is around 510ºC/950ºF for System 96; 516ºC/962ºF for Bullseye and Uroboros for example. The range for a fusing glass will be around 549ºC to 477ºC/1020ºF to 890ºF for fusing glasses. Although the upper half of that range is merely theoretical. The lower end of the range is the strain point.

The annealing soak is to equalise the temperature throughout the glass to within 5ºC. Once the annealing soak is complete, the first stage of cooling begins. This first 55ºC/100ºF below the annealing soak is essential to the adequate annealing of the glass.  And this illustrates the impracticality of annealing in the upper part of the range.  The first cool rate needs to be maintained to at least 55ºC/100ºF below the low end of the annealing range.

To exemplify this. It would be possible to start the annealing at about 550ºC/1020ºF for any of these glasses. But the slow rate of decline in temperature, following the equalisation soak, would need to be maintained for the whole range of 550ºC/1020ºF to 429ºC/805ºF, rather than just the 55ºC/100ºF from the anneal soak point. This would more than double the annealing cool time. This high temperature anneal is a much slower process, which – together with the more rapid relief of stress at the annealing point – is why the top of the range is never used for the temperature equalisation point. It is also why the Spectrum 96 soak above the annealing point was not essential.

Soak

The annealing point is the temperature at which, if all the glass is at the same temperature, the most rapid cooling can take place. To achieve that equalisation temperature (+ or – 5ºC throughout), the glass needs to be soaked at the annealing point for varying lenghts of time relating to thickness and other variables. To complete the anneal and keep the glass within that tight range of temperature, the anneal cool needs to be continued at a steady slow rate.

Lower part of annealing range

Bullseye now recommends the use of 482ºC/900ºF for  the temperature equalisation soak, but have increased the soak time from 30 minutes to one hour. Choosing to start the annealing process at the lower part of the annealing range speeds the process for thick slabs and is very conservative for thinner glass. Bullseye have not changed the composition of their glass so the anything annealed at 516ºC/960ºF for things 6mm/0.25" or less is still properly annealed.

Using the bottom end of the annealing range for thick items, means there are a fewer number of degrees of very slow cooling to the strain point. But this lower soak, or temperature equalisation point, requires a longer soak to equalise the temperature within the glass before the slow steady decline in temperature to maintain the temperature differentials within the glass to less than 5ºC.

Bullseye have found that using a temperature a bit above the bottom end – 482ºC/900ºF – with a long soak reduces the total time in the kiln, but continues to give a good anneal. In the case of Bullseye, 461ºC/863ºF is the bottom end of the annealing range according to the calculations indicated above. 

Zinc Health and Safety

So much is said about the toxicity of zinc, I thought to look up some facts.

As there is significant concern about health issues, it is useful to look in detail at the health and safety issues around the use of zinc at elevated temperatures.  Zinc is absorbed into the body by inhalation of fumes and consumption of zinc containing materials.

Toxicity

Although zinc is an essential requirement for good health, excess zinc can be harmful. Excessive absorption of zinc suppresses copper and iron absorption … [which results in the symptoms of zinc intoxication].  Stomach acid contains hydrochloric acid, in which metallic zinc dissolves readily to give corrosive zinc chloride. … The U.S. Food and Drug Administration states that zinc damages nerve receptors in the nose, causing [loss of smell].

Evidence shows that people taking 100–300mg of zinc daily may suffer induced copper deficiency. … Levels of 100–300mg may interfere with the utilization of copper and iron or adversely affect cholesterol. … A condition called the zinc shakes or "zinc chills" can be induced by inhalation of zinc fumes while brazing or welding galvanized materials. 

https://en.wikipedia.org/wiki/Zinc

Poisoning

Consumption of zinc can result in death, but requires large amounts (over 1 kg in one case).  Smaller amounts result in lethargy and gross lack of coordination of muscle movements or apparent intoxication. https://en.wikipedia.org/wiki/Zinc

Research and W.H.O. Information

The Essential Toxin: Impact of Zinc on Human Health, by Laura M. Plum, Lothar Rink, and Hajo Haase^*

Compared to several other metal ions with similar chemical properties, zinc is relatively harmless. Only exposure to high doses has toxic effects, making acute zinc intoxication a rare event. In addition to acute intoxication, long-term, high-dose zinc supplementation interferes with the uptake of copper. Hence, many of its toxic effects are in fact due to copper deficiency. While systemic [balance] and efficient regulatory mechanisms on the cellular level generally prevent the uptake of [cell destructive] doses of [environmental] zinc, … zinc [within the body] plays a significant role in cytotoxic [death of individual cells] events in single cells. … One organ where zinc is prominently involved in cell death is the brain, and cytotoxicity in consequence of [inadequate blood supply] or trauma involves the accumulation of free zinc.

Rather than being a toxic metal ion, zinc is an essential trace element. Whereas intoxication by excessive exposure is rare, zinc deficiency is widespread and has a detrimental impact on growth, neuronal development, and immunity, and in severe cases its consequences are lethal. Zinc deficiency caused by malnutrition and foods with low bioavailability, aging, certain diseases, or deregulated homeostasis [equilibrium] is a far more common risk to human health than intoxication.

Conclusions

Zinc is an essential trace element, and the human body has efficient mechanisms, both on systemic and cellular levels, to maintain [balance] over a broad exposure range. Consequently, zinc has a rather low toxicity, and a severe impact on human health by intoxication with zinc is a relatively rare event.

Nevertheless, on the cellular level zinc impacts survival and may be a crucial regulator of [the death of cells occurring as a normal and controlled part of an organism's growth or development]  as well as neuronal death following brain injury. Although these effects seem to be unresponsive to nutritional supplementation with zinc, future research may allow influencing these processes via substances that alter zinc [balance] instead of directly giving zinc.

Whereas there are only anecdotal reports of severe zinc intoxication, zinc deficiency is a condition with broad occurrence and potentially profound impact. Here, the application of “negative zinc”, i.e., substances or conditions that deplete the body of zinc, constitute a major health risk. The impact ranges from mild zinc deficiency, which can aggravate infections by impairing the immune defence, up to severe cases, in which the symptoms are obvious and cause reduced life expectancy.  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2872358/

Zinc came
Credit: leadandlight.co.uk

World Health Organisation Document

10.2.2 Occupational exposure

Occupational exposure to dusts and fumes of zinc and zinc compounds can occur in a variety of settings in which zinc is produced, or in which zinc and zinc-containing materials are used. Typical airborne exposures observed include 0.19–0.29 mg/m3 during the smelting of zinc-containing iron scrap, 0.90–6.2 mg/m3 at non-ferrous foundries and 0.076–0.101 mg/m3 in hot-dip galvanizing facilities. Far higher exposures are possible during particular job activities, such as welding of zinc-coated steels in the absence of appropriate respiratory protection and/or fume extraction engineering controls.

Occupational exposure to high levels of zinc oxide and/or nonferrous metals is associated with metal-fume fever. [a condition in which the sufferer has influenza type symptoms - a raised temperature, chills, aches and pains, nausea and dizziness. It is caused by exposure to the fume of certain metals - commonly zinc].  This is usually a short-term, self-limiting syndrome…. Induction of metal-fume fever is most common with ultra-fine particles capable of deep lung penetration under conditions of exposure. Studies on volunteers conducted under short-term exposure conditions (77–153 mg/m3 for 15–30 min) have detected pulmonary inflammation responses (including [inflammation] induction) which are consistent with manifestations of metal-fume fever and support an immunological [cause] for this acute reversible syndrome.

Evaluation

Based on the available information, it is not possible to define a no-effect level for pulmonary inflammation from exposure to zinc oxide fume.

10.2.4 Risks of zinc excess

Toxic effects in humans are most obvious from accidental or occupational inhalation exposure to high concentrations of zinc compounds, such as from smoke bombs, or metal-fume fever. Modern occupational health and safety measures can significantly reduce potential exposure. Intentional or accidental ingestion of large amounts of zinc leads to gastrointestinal effects, such as abdominal pain, vomiting and diarrhoea.

In the case of long-term intakes of large amounts of zinc at pharmacological doses (150–2000 mg/day), the effects (sideroblastic anaemia [inability to make haemoglobin], leukopenia [low white cell quantities] and hypochromic microcytic anaemia [iron deficiency]) are reversible upon discontinuation of zinc therapy and/or repletion of copper status, and are largely attributed to zinc-induced copper deficiency.

High levels of zinc may disrupt the [balance] of other essential elements. For example, in adults, subtle effects of zinc on copper utilization may occur at doses of zinc near the recommended level of intake of 15 mg/day and up to about 50 mg/day. Copper requirements may be increased, and copper utilization may be impaired with changes in clinical chemistry parameters, but these effects are not consistent and depend largely upon the dietary intake of copper. Distortion of lipoprotein metabolism and concentrations associated with large doses of zinc are inferred to be a result of impaired copper utilization. In groups with adequate copper intake, no adverse effects, with the exception of reduced copper retention, have been seen at daily zinc intakes of [less than] 50 mg/day. There is no convincing evidence that excess zinc plays a [casual] role in human carcinogenesis. The weight of evidence supports the conclusion that zinc is not genotoxic [damaging of genetic information in cells] or teratogenic [affecting the development of embryos]. At high concentrations zinc can be cytotoxic [toxic to cells].   https://www.who.int/ipcs/publications/ehc/221_Zinc_Part_3.pdf?ua=1

zinc sheet 
Credit: Belmont Metals

Use and Risks of Zinc in Kilnforming

Zinc melts at 420°C and boils at 907°C, so any fumes will be emitted only around and above the full fusing temperature of glass.

The main problem in kilnforming is that the metal melts at such a low temperature that it is not useful for containing the glass.

There is anecdotal evidence to indicate that firing zinc contaminates the kiln, leading to subsequent devitrification issues.  This can be cleared by firing bentonite at high temperature in the kiln to absorb the zinc.

It is not a high-risk metal, even if it were to vaporise (above 900°C).

Research papers show zinc poisoning to be extremely rare. It is usually associated with taking too large daily doses of zinc as a dietary supplement, or swallowing USA pennies - made largely of zinc - which dissolves in stomach acid and creates large problems for the digestive system.  Where zinc intoxication occurs, it is largely reversible.

Conclusion

The idea that zinc will poison you in kilnforming conditions is simply not correct.

Quoting for Fused Glass Commissions

When quoting on a fused glass commission, what are all the factors to consider?

Commission for Glasgow University

Quote the same way as for leaded or copper foil.  But if you don’t work in those forms, that statement will not be much help.

The elements to consider are:

·        Design time and value (making sure you retain the copyright of the design).

·        Amount of time to assemble. You need to think clearly about how long it really takes.  You need to be charging a reasonable amount for your time. Think about skilled trades people’s charges and that you have additional artistic skills.

·        Amount of glass to be purchased (rather than used) to make the piece, even if much is from stock – you must replace it after all.

·        Number and cost of kiln firings.  Be clear about how many firings might be required, if something does not work out first time.  Be clear about how much each firing costs including depreciation on the kiln.

·        Incidental supplies.  All the little things that are necessary to supply your practice, such as art materials, kiln supplies, etc.

·        Overheads. This is the cost to run your practice.  If the studio is part of the home premises, add a proportion of the running expenses of the house to the cost.  The cost of business - advertising, promotion, printing, etc., all need to be included.

·        Profit. You do need to make a profit to stay in business. Decide what that is and add that percentage to the cost.

·        Allowance for contingencies (20% of the price already determined is usual).

·        Delivery/installation costs (normally in addition to the cost of design and making).

It is advisable to find out what the client’s budget for the commission is before starting any designing.  If it is too small for their specification, decline the commission.  Otherwise, you can design to the budget.  A large budget allows expansive or highly detailed works.  A small budget restricts the size or detail possible.

Some people charge more for a commission. Some, like me charge less, as I am getting most of the money up front, rather than maybe sometime in the future.  Cash is important.

Some artists take 1/3 to make the design, 1/3 on approval of design, and final 1/3 on completion. This is widely used in the interior design field. You may want to consider requiring a non-refundable deposit of one third to make a start and the remaining two thirds on completion as an alternative. 

A contract of some sort is essential.  It needs to cover the expectations of both parties.  Cost, of course.  When is it to be completed? Requests for colours, shapes, location, style, etc.  If the client wants approval at various stages, you need to either state what these stages are, or more sensibly, decline the commission. 

The contract does not have to be legalistic.  It can be a letter stating the terms of the commission that is sent by you to the client and acknowledged by them.

Determining the price for a commission requires consideration of the costs of time and materials, and the values of what you do.  A contract of some sort is required. It can be a simple letter with a statement of the agreed conditions.

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).

Single Layer Circle with Decorative Rim

A question arose:

If you fuse a single 20cm diameter sheet of 3mm glass to full fuse, [with a decorative rim] what happens? … Would the lack of two layers in the centre be a problem for the 6mm rule?

This layup risks trapped air and a large central bubble.  The explanation involves the combination of volume control and weight.

Volume control

The volume control relates to the single 3mm layer in the centre.  The glass will thin in the centre and thicken at the perimeter.  This leads to the risk of thinning to the degree that bubbles are created in the centre.  The edges will also draw in as the viscosity - surface tension - of the glass pulls the glass toward a 6mm thickness.

Weight

The explanation is also about weight.  The decorative rim adds weight to the outside of the piece.  This weight will “seal” the rim of the glass to the shelf, reducing the possibility of air escaping from under the central portion of the piece.  This weight effect on the rim increases the risk of a large central bubble.

Profile

Another influence on the result of the fuse is the degree of fuse.  At full fuse the viscosity of the glass is less and so resists the force of expanding air much less than when cooler. Even at rounded tack fuse, the glass will be unable to resist the formation of bubbles. As the glass thins and viscosity decreases, any air at all will cause a bubble.

Changes for the future

Avoidance of bubbles in this piece relate to design, scheduling and technique.

Design

It is possible to design a piece of this nature to avoid the volume control issue.  The base piece could have a smaller circle or rectangle centralised on top inside the proposed perimeter.  The rim can then have the decorative elements placed.  If they are spaced widely, frit can be used to fill significant gaps.  The piece can then be placed in the kiln for a full fuse.

Scheduling

You can also fire the piece as originally described very slowly to a low temperature.  This uses the concept of heat work. By applying the heat over a long period, you can achieve the same effect as would be achieved by a faster rate of advance to a higher temperature. 

There are at least two ways to increase the heat work.  You can use a very slow rate of advance to a point slightly above the softening point of the glass.  This will be the lower end of the slumping temperature range of your glass.  The soak may be for hours.  You will need to observe when the effect you want is achieved.

You also can choose the same lower slumping temperature and reach it in your standard fashion.  This will require an even longer soak time to achieve the same result.

In both these low firing approaches, you will need to observe to determine when the piece is finished.

Technique

The “flip and fire” technique may also work on the single layer with an added rim.  To do this you build the piece upside down on the shelf.  It helps to draw an outline of design on Thinfire, or Papyros.  Place the decorative elements and cap them with the clear.  Take the whole to a rounded tack fuse.  When cool, clean well and fire to a tack fuse again.  This will give something less than a full fuse, but it will be more than a tack, as the heat work is cumulative.

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

Summary

A single layer piece with a decorative rim is most likely to produce bubbles in the centre.  There are some ways to overcome this: design, scheduling, and technique. Design is the most likely to be successful.

Bubble formation

Question:

I had 2 kiln loads where every piece had huge bubbles. The 3rd time I did a test fire and put 2 pieces on Thinfire and one on the kiln shelf. The one without thin fire got a bubble. Theories?

Response:

Your experiment showed that the Thinfire prevented the bubbling.  The question you are asking is why.

Fibre paper is a porous material allowing air to move through it and from under the glass.  If the shelf has only slight depressions, the Thinfire or Papyros will allow air out from under the glass avoiding bubbles.

Thicker fibre paper can provide a different and more level surface if it is thick enough to span any depressions in the shelf, while allowing air out too.

First, it is apparent that your shelf is not absolutely smooth and level.  This has been shown by your experiment where Thinfire prevented bubbles where previously there had been bubbles.  It showed that without the additional cushion that the depressions, although slight are enough to cause bubbles without additional fibre paper separators.

Second, although you do not give your schedule, the firing is too hot.  There is dog boning of your thin glass.  The bubbles on the thinner glass have burst and thinned greatly.

Firing hot causes the glass to become much less viscous than needed to perform a full fuse and allows the trapped air to push bubbles into and through the glass.  Lower temperatures with longer soaks/holds enable the glass to better resist the formation of large bubbles.  Also firing more slowly enables air to escape and allows the use of lower temperatures while still being able to achieve the fuse you want.

More information is available in Low Temperature Kilnforming, an evidenced based guide to scheduling.