Refiring and Annealing

A question about re-fusing: 

I have just taken a large piece, with uneven layers out of the kiln, it went in … and fired for double thickness. A small piece has flipped and is showing the white side. … If I cover this with a thin layer of coloured powder frit, does the piece need the long anneal process when I fire it again, please. I will be taking it up to the lowest tack fuse temperature possible [my emphasis], so the rest doesn’t change too much.

When considering the re-firing of a fused piece, even with minimal changes, the schedule needs re-evaluation of both ramp rates and annealing. In this case, the major change is using a sinter firing – “the lowest tack fuse temperature possible”.

Ramp Up Rates

Previously the piece was in several layers.

• The piece is now a thicker single piece and needs more careful ramp rates.
• It is also of uneven thicknesses.
• And you intend to fire to a sharp tack or sinter.

These things make a requirement for more cautious firing. You cannot fire as quickly from cold as forthe original unfired piece. Previously, the sheets could be heated as though separate. They were not hot enough to stick together until beyond the strain point. They now could experience the differential expansion from  rapid heating, which can cause breaks. 

The previously fired piece will need a slower initial ramp rate this time. This is because you are firing for a sharp tack. This is also known as fusing to stick, or sintering. It is not because of a second firing. It is because of the differences in the glass for this firing. You are firing a single thicker piece of uneven layers to a sharp tack.

Looking at Stone* and the Bullseye chart for Annealing Thick Slabs indicates that in general, the first ramp rate should be halved for each doubling of calculated thickness. This is for full fused items. However, this is going to be a more difficult fusing profile - sintering. The calculation for sintering is as for 2.5 times the thickest part of the piece. This factor of 2.5 was determined by a series of experiments that are detailed in the eBook Low Temperature Kilnforming.

You started with firing two layers of 3mm/0.125” at possibly 330°C/595°F. You are now firing the fused 6mm/0.252 piece to a sharp tack. This means you should be looking at firing for 2.5 times or 15mm/0.625”. This implies 240°C/435°F as the maximum first ramp rate. A more cautious approach is to fire to 300ºC/540ºF at a rate of 72ºC/130ºF, as most heat-up breaks occur below that temperature. You should maintain that rate to 540°C/1005°F afterwards. 

Annealing

The annealing time and cool rate will be affected in the same way as the change to a sharp tack firing. Without that fuse profile change, and no change in the profile or thickness of the piece, it could have been annealed as previously. However, changing to a sharp tack means a longer anneal soak is required. This sharp tack annealing is for 2.5 times the thickness or 150 minutes.

Cooling

The cooling rates for this piece are not the same as for the first firing. A sharp tack firing will require cooling rates of:

• 40ºC/73ºF to 482°C/900°F.
• 72ºC/130ºF f427ºC/800ºF.
• 240ºC/435ºF to room temperature

This applies regardless of the fusing glass you are using, as it is the viscosity which is the important factor in cooling.  Viscosity is primarily related to temperature.

Refiring with Significant Additions.

Ramp rate

If there are additions to the thickness, a slower first ramp rate will necessary. If an additional 3mm layer is placed on top of a 6mm base for a rounded tack, you will need to schedule as for 19mm/0.75” (twice the thickest part). This will be 150°C/270°F for the first ramp rate. For a sharp tack, it will be as for 22.5mm/0.825”. The maximum rate will be reduced to 120ºC/216F for the first ramp. This shows the additional caution required for sharper fusing profiles.

Annealing

The annealing will need to be longer than the first firing. The thickness has changed with the additions of pieces for a rounded tack firing. Instead of annealing for 6mm/0.25” you will be annealing as for 19mm/0.75”. This requires a hold of three hours at the annealing temperature and cooling over three stages:

• The first cool rate is 25°C/45°F per hour to 482°C/900°F.
• The second rate is 45°C/81°F per hour to 427ºC/800ºF.
• The last rate is at 90C°C/162°F per hour to room temperature.

If there are additions, plus firing to the lowest possible tack temperature – as in the example - the firing must be as for 2.5 times the actual thickness. Annealing as for 25mm/1” gives rates of:

• The first cool rate is 15°C/27°F per hour to 482°C/900°F.
• The second rate is 27°C/49°F per hour to 427ºC/800ºF.
• The last rate is at 90C°C/162°F per hour to room temperature.

These examples show how dramatically later additions in thickness can add to the length of the firing to re-fire a well-annealed piece without breaking it on the heat-up. It also shows that changing the profile to a sharper tack affects the annealing and cooling times and rates.

 

*Graham Stone. Firing Schedules for Glass; the Kiln Companion. 2000, Melbourne. ISBN 0-646-397733-8

As a side note Stone’s book has become a collectable.

Go-to Schedules

 It’s a schedule I always use.

This is a frequent statement in response to a firing that has gone wrong.

You don't always fuse the same thing, or the same design, or the same thickness, etc. So why always use the same schedule?

The schedule for the firing each piece needs to be assessed individually. It may be similar to previous firings. But it may have differences. Assess what those differences mean for the firing.  Some factors to consider.

Addition of another layer to a stack in tack fusing makes a difference to the firing requirements. Even if it is only on part of the piece. It needs to have a slower ramp rate and a longer anneal soak and slower cooling.

A different design will make a difference in firing requirements too. For example, if you are adding a design to the edges of the glass, you will need different bubble squeeze schedules than when you do not have a border. It will need to be slower and longer than usual.

The placement of the piece in the kiln may require a re-think of the schedule too. If the piece is near the edge of the kiln shelf, or in a cool part of the kiln while others are more central, the same schedule is unlikely to work. You need to slow the schedule to account for the different heat work each piece will receive during the firing.

If you have introduced a strong contrast of colour or mixed transparent and opalescent glass in a different way, you may need slower heat ups and longer cools.

These are some examples of why the same schedule does not work all the time. It works for pieces that are the same. But it does not work for pieces that are different. And we should not expect it to.

There are sources to help in developing appropriate schedules. Bob Leatherbarrow’s book FiringSchedules for Kilnformed Glass is an excellent one.

Another one is especially good for lower temperature work: Low Temperature Kilnforming, anEvidence-Based Approach to Scheduling. Be aware that I have a vested interest here – I wrote it.

 

 

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.

Comparison of Citric Acid and Trisodium Citrate.

These two substances are useful means of removing kiln wash and refractory mould material from glass. They are important where abrasive methods such as sand blasting are not available or appropriate.

My recent experience with both citric acid and trisodium citrate shows differences in performance. This makes each more suitable in different contexts.

credit: Amazon

Trisodium citrate is the safest option when long soaks are required to remove refractory mould material. The trisodium citrate removes any risk of etching the glass on long soaks. It has been shown by Christopher Jeffree that two-day soaks in this will not etch the glass. It is most suitable for casting work.

Items cleaned with citric acid and vinegar
credit: Christopher Jeffree

Citric acid acts quickly on kiln wash, making long soaks less necessary. Depending on the thickness of the stuck kiln wash and the amount of agitation of the stuck kiln wash, the time required may be only a dozen minutes. It rarely takes more than a few hours.  Citric acid does not work quickly on refractory materials. This makes the trisodium citrate the better choice for long soaks.

 More on citric acid as a cleaner

 More on citric acid

More on trisodium citrate