Time and Temperature

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“What are the pros and cons on turning up the max temperature slightly Vs. a longer hold time”? Lea Madsen

This is a difficult question to answer, because there are variables such as

the temperature range,

the ramp rates, and soaks,

the forces acting upon the glass at a given temperature, 

the process,

the desired outcome of the firing,

etc. 

When talking about temperature vs. time, it is heat work that we are considering.  In many processes time and temperature are interchangeable, although the temperature range is important too.  This is a brief discussion of heat work in various processes.

Slumps

Slumping temperature is generally in the range of 620˚C-680˚C/1150˚F -1255˚F *, which is below the devitrification range.  This allows the exchange of time for temperature without risk, allowing more time rather than more temperature.  Higher temperatures cause more marking from the mould since the bottom of the glass is softer than at lower ones.  Lower temperatures give higher viscosity, so the glass is stiffer, resisting marks.

Low temperature fuses

Sharp tack fusing, freeze and fuse, some pate de verre processes, and sintering occur in the 650˚C -720˚C /1150˚F - 1320˚F range, risking devitrification only at the upper end of this range.  Extending the time rather than the temperature is important to maintain detail in these processes.  Higher temperatures will smooth the surface, risking loss of detail.  

Rounded tack processes (720˚C – 760˚C /1320˚F - 1400˚F)

These are within the devitrification range making the choice between time and temperature a balance of risks.  In my experience, it takes about an hour for visible devitrification to develop.  This means that you can extend the time, if the total time between the end of the bubble squeeze and the working temperature, including the hold time, is less than an hour.  It has the advantage of a more secure attachment between the pieces of glass, without altering the surface much. 

But if extending the soak time increases the time in the devitrification zone to be more than an hour, it is advisable to increase the temperature, rather than time.  Devitrification develops in the presence of air, so reducing the time in that range reduces the risk of devitrification developing.  The glass is moving during rapid ramp rates, reducing the chance of devitrification.

Drops

This includes drapes, and other free forming processes.  Kilnformers will be observing the progress of these firings, making it easier to balance temperature and time.  There are already long holds scheduled for the processes, so it is a matter of getting the right temperature.  If, after half an hour at the scheduled top temperature, the glass has not moved much, it is time to increase the temperature by, say 10˚C/18˚F and observe after another half hour, repeating the temperature increase if necessary.   Be aware of thinning the glass at the shoulder by setting a high temperature.  Free drops may take as much as 6 – 8 hours, so patience and observation are important to get good results.

Full fuse

At full fuse try to get the work done in 10 minutes to avoid complications with devitrification.  So, increasing the temperature rather than the length of the soak seems best.

Flows

Whether frit stretching, making pattern bars, pressing, etc., low viscosity is important.  Viscosity is closely related to temperature, so increasing the temperature is the better choice.  Increasing time without increasing temperature does not change viscosity much.

Casting

Extending time at top temperature seems best for open face casting, as the temperature is already high.  A slow ramp rate to that top temperature may make adding time unnecessary, because the heat work will be increased by the slow rise.  Experience has shown that a rate of 200˚C/360˚F is enough to avoid devitrification.  With enclosed castings devitrification is not such a risk, so time can be added without concern.

 

Observation

In all these processes it is advisable to observe the progress of the firing by quick peeks to determine the effective combination of temperature and time.  Also note that heat work is cumulative, making for changes in profile with repeated firings. 

 

* The softening point of float glass is around 720°C/1328°F, so the slumping range is about 700°C/1292° to 750°C/1382°F.

Flows

 

Credit: Marcy Berman

I have not had much success [with] the Patty Gray mould despite using the recommended firing schedule. I always have holes or bubbles and the edges are not smooth.

The schedule for Oceanside was:

• 111°C/200°F per hour to 537°C1000°F for 15 minutes
• 167°C/300°F per hour to 662°C/1225°F for 30 minutes
• 195°C/350°F per hour to 798°C/1470°F for 20 minutes
• 9999 to 510°C/950°F for 120 minutes
• 55°C/100°F per hour to 371°C/700°F off

 Your picture shows a bottom view of the piece - made of cullet pieces - as fired. Two large bubbles show to have been created from the bottom rising through the glass to the top.

 Although a long bubble squeeze will not prevent this, it will help to reduce the number of bubbles, and especially large ones. Because of the number of pieces and the thickness of the glass put into the mould, a longer bubble squeeze would benefit this piece.

 The bubble squeeze can be as you have done this – at a single temperature – with a soak. In this case, I would have used 60 to 90 minutes as the soak.

 The other bubble squeeze method is to start the squeeze about 55C/100F below the top of the bubble squeeze. Most people use a soak of about 30 minutes there. They then proceed at a rate of between 30C/55F and 55C/100F to the top of the bubble squeeze and soak there for another 30 minutes. The rates and soak times will vary according to the thickness or complexity of the piece.

 I dispense with the soak at the beginning of the bubble squeeze on the grounds that at 610/1130F so little movement will be created that it is a waste of time. I would prefer to have a slower ramp rate to the top temperature and a longer soak there. I know the glass will be moving at those temperatures. Many people find the soak at the beginning of the bubble squeeze successful.

 The schedule to the top of the fuse is faster than the rest of the schedule. When I want a piece to flow, and especially, to fill gaps, I slow the rate. In this case a rate of between 100C/180F and 167C/300F would be slow enough to allow the glass to flow to fill gaps.

 I want to ensure the glass has enough time when it is flowing most freely at the top temperature to level out. This requires scheduling a longer soak at the top and observing how well the glass is levelling out. If more time is required you can add it on the “run,” and advance to the next segment when the surface is as wanted. Read up in your kiln manual how to do both these things.

 Yes, the rate is one which will enable devitrification to form on flat glass. The soak at top temperature is even more likely to promote it. However, as the glass is flowing, less devitrification has an opportunity to form. The crystallisation – which is what devitrification is - of the glass takes time to form. The movement of the glass surface is sufficient to reduce the formation of those crystals. It is of course likely there will be some devitrification, but not as much as the slow rates and long soaks would lead you to think. 

 But for these flows there always is the possibility of devitrification. You have to plan a method of removing it. Unless the surface is very flat, grinding the top is not a fast way to remove it. Sandblasting is a quick way to remove devitrification. Another way is to sift a thin layer of clear glass powder over the surface. This is an increasingly popular way to deal with devitrification for those without access to sandblasting facilities. When fired again, the powder melts and forms a new shining surface. The piece will need to be fired fire again whether sandblasted or covered in glass powder.

The summary for flows:

• Slow down to top temperature.
• Give sufficient time there to get the flow needed.
• Observe the progress as you near the top temperature.
• Extend the soak or advance to the next segment when the surface is smooth.
• Anneal soak for the calculated thickness.
• Use a three-stage cool – as outlined in the Bullseye chart for annealing thick slabs - to ensure no temporary contraction stresses are created.
• Accept there will be devitrification.