Tuesday, 14 October 2025
Observations on Some Suggestions about Annealing
Wednesday, 17 September 2025
A Sintering Project
The project is to fire 6mm “balls” stacked 3 high onto a single sheet of clear glass without significant alteration to the base sheet or to the stacked balls. This creates a total thickness of 21mm. The proposal is to sinter the whole in one firing.
Scheduling for a sinter firing needs to be done as though 2.5 times the thickest part – in this case 52mm, or 2 inches
It is slightly more risky to do this in two firings, than one, in my opinion. A suggested schedule for sintering frit using Bullseye was:
- 100ºC /180ºF — 482ºC /900ºF, 60' =5.8 hrs
40ºC /72ºF — 593ºC/1100ºF,10' =2.8
20ºC /36ºF — 665ºC /1230ºF,30 =4.1
Skip to anneal temperature, soak for 6 hours =6.5
6.7ºC /12ºF — 427ºC /800ºF,0' =8.2
12ºC /22ºF — 371ºC /700ºF,0' =4.7
40ºC /72ºF — room temperature,0’ =8.8
Off =40.9 hours total or 1.7 days
This was annealing as for 38mm/1.5 inches thick. Annealing for 50mm/2” thick would need about 112 hours or 4.6 days.
However this schedule was not successful – the pieces were only lightly stuck together. Thinking about why, led to the proposal that the soak time and temperature were not long or high enough to give adhesion between the pieces.
A second attempt used a faster ramp rates to higher temperatures.
200°C /360°F – 540°C /1004ºF, 30’ =3.2 hrs
60°C /108°F -625°C /1157ºF, 30’ =1.92
30ºC /54ºF - 685ºC /1265ºF, 120’ =4.0
skip to anneal temperature and soak/hold for 4 hours (as for 25mm/1”)
15ºC /27ºF – 427ºC /800ºF, 0’ =3.67
27ºC /49ºF – 370ºC /700ºF, 0’ =2.11
90ºC /162ºF – 50ºC /122ºF, 0’ =3.56
Off
= a minimum total of 18.5 hours plus natural cooling of the kiln
This schedule used a:
faster first ramp to a higher (540ºC /1004ºF) first soak
a faster (60ºC /108ºF, which is 150% of the previous) rate to the lower slump temperature (625ºC /1157ºF)
the same relative reduction (50%) in rate to a higher temperature (685ºC /1265ºF)
a shorter (120’) anneal soak
and consequently faster cooling rates, which showed no stress after firing
The whole structure held together and was sound. There was no apparent change in the size of the individual 6mm balls.
This difference in scheduling is an illustration of how time and temperature can be interchanged.
It also shows that size matters when sintering pieces together. Higher temperatures and more time are required for dots and balls than for frit.
More information is available in my e-book Low Temperature Kilnforming, available from Bullseye, Etsy and stephen.richard43@gmail.com
Wednesday, 13 August 2025
Devising Slumping Schedules
A while ago Bob Leatherbarrow gave a presentation to Lunch With A Glass Artist (LWAGA) on slumping schedules. You can follow a recording of the Zoom meeting after joining the Facebook group: Lunch With A Glass Artist – Larry Pile.
The most important point for thinking about the process he brought up is his order of consideration of factors. They are:
- span
- thickness
- viscosity
How big is the unsupported part of the glass. Glass on larger span moulds will begin slumping at lower temperatures.
The thickness has an effect. With the same ramp rate thicker glass will need higher temperature.
The viscosity of the glass also affects the temperature of the slump. Low viscosity glass will slump at lower temperatures than higher ones, e.g., black vs. white.
Then you can begin to think about temperature and time. The objective is to use the lowest temperature to get the slump done in 30 minutes, so there is no marking of the glass touching the mould.
There is a lot more in the presentation to LWAGA. Join the Facebook page by answering two questions to get access.
There is even more information about fusing principles and practices in his book FiringSchedules for Kilnforming, Just Another Day at the Office. This inexpensive eBook is worth much more than the purchase price!
A lot of information is also contained in my e-book Low Temperature Kilnforming, available from Bullseye, Etsy and from Stephen.richard43@gmail.com
Wednesday, 16 July 2025
Devising Slumping Schedules
A while ago Bob Leatherbarrow gave a presentation to Lunch With A Glass Artist (LWAGA) on slumping schedules. You can follow a recording of the Zoom meeting after joining the facebook group Lunch With A Glass Artist – Larry Pile.
The most important point for thinking about the process he follows is the order of slumping factors. They are:
span
thickness
viscosity
How big is the unsupported part of the glass? Glass on larger span moulds will begin slumping at lower temperatures than on smaller spans.
The thickness has an effect. With the same ramp rate thicker glass will need higher temperature or longer time.
The viscosity of the glass also affects the temperature of the slump. Low viscosity glass will slump at lower temperatures than higher ones, e.g., black vs. white.
Then you can begin to think about temperature and time. The objective is to use the lowest temperature to get the slump done in 30 minutes to avoid marking of the glass touching the mould, leaving a smooth shiny back.
There is a lot more in the presentation to LAWAGA. Join the facebook page to get access.
There is even more information about fusing principles and practices in his book Firing Schedules for Kilnforming, Just Another Day at the Office.
https://www.leatherbarrowglass.com/purchase/firing-schedules-for-kilnformed-glass
This inexpensive eBook is worth much more than the purchase price!
Sunday, 23 February 2025
Rapid Ramp Rates with Soaks
In both these cases, these are about the materials holding or contained in the glass, rather than the glass itself.
Wednesday, 29 January 2025
Tack Fusing Considerations
Initial Rate of Advance
Tack fuses look easier than full fusing, but they are really
one of the most difficult types of kiln forming. Tack fusing requires much more
care than full fusing.
On heat up, the pieces on top shade the heat from the base glass leading to
uneven heating. So you need a slower heat up. You can get some assistance in
determining this by looking at what the annealing cool rate for the piece is. A
very conservative approach is needed when you have a number of pieces stacked
over the base layer. One way of thinking about this is to set your
initial rate of advance at approximately twice the anneal cool rate.
Annealing
The tacked glass us loosely attached rather than fully formed together. So, the glass pieces are still able, partially, to act as separate entities, meaning excellent annealing is required.
Effects of thicknesses, shapes, degree of tack
- Tack fusing of a single additional layer on a six millimetre base
- Rectangular pieces to be tack fused
- Sharp, pointed pieces to be tack fused
- Multiple layers to be tack fused
- Degree of tack – the closer to lamination, the more time required
Glass contracts when it's cooling, and so tends to pull into
itself. In a flat, symmetrical fuse this isn't much of a problem. In tack fuses
where the glass components are still distinct from their neighbours, they will
try to shrink into themselves and away from each other. If there is not enough time for the glass to
settle into balance, a lot of stress will be locked into the piece that either
cause it to crack on cool down or to be remarkably fragile after firing. In tack fusing there also are very uneven
thicknesses, making it is hard to maintain equal temperatures across the glass.
The tack fused pieces shield the heat
from the base, leading to localised hot spots during the cool down.
On difficult tack fuses it's not unusual to anneal for a
thickness of two to three times greater than the thickest part of the glass. That extended cool helps ensure that the glass
has time to shift and relax as it's becoming stiffer, and keeps the temperature
more even throughout.
In general, tack fused pieces should be annealed as though
they are thicker pieces. Recommendations range from the rate for glass that is
one thickness greater to at least twice the maximum thickness of the whole
item. Where there are right angles -
squares, rectangles - or more acutely angled shapes, even more time in the
annealing cool is required.
It must be remembered, especially in tack fusing, that
annealing is much more than the annealing soak. The soak is to ensure all the glass is at the
same temperature, but it is the anneal cool that ensures the different
thicknesses will all react together. That
means tack fusing takes a lot longer than regular fusing.
The more rectangular or pointed the pieces there are in the
piece, the greater the care in annealing is required. Decisions on the schedule to use varies - some
go up two or even four times the total thickness of the piece to choose a
firing schedule.
A simple way to determine the schedule is to subtract the
difference between the thickest and the thinnest part of the piece and add that
number to the thickest part. If you have a 3mm section and a 12mm section, the
difference is 9mm. So, add 9 to 12 and get 17mm that needs to be annealed for.
This thickness applies to the heat up segments too.
Another way to estimate the schedule required is to increase
the length the annealing schedule for any and each of the following factors:
The annealing schedule to be considered is the one for at
least the next step up in thickness for each of the factors. If you have all
five factors the annealing schedule that should be used is one for at least
21mm thick pieces according to this way of thinking about the firing.
4 – Testing/Experimentation
The only way you will have certainty about which to schedule
to choose is to make a mock-up of the configuration you intend in clear. You can then check for the stresses. If you have chosen twice the thickness, and
stress is showing, you need to try 3 times the thickness, etc., which can be done
on the same piece. You can reduce time
by having your annealing soak at the lower end of the annealing range (for
Bullseye this is 482C, rather than 516C).
You will need to do some experimentation on what works best
for you. You also need to have a pair of polarisation filters to help you with
determining whether you have any stress in your piece or not. If your piece is
to be in opaque glasses, The mock-up in clear will be useful.
First published 18.12.2013
Revised 29.01.25
Over Annealing
- · Lengthy anneal soaks can induce stress in certain circumstances. More later.
- · Excessive annealing soaks waste time and money.
- · Annealing is more than the soak. It is a combination of equalisation of the heat within the glass (not just temperature) and the gradual cooling of the glass to below the lower strain point to ensure the glass does not incorporate differences of temperature of plus or minus 5°C.