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Large Flat Panel - Breakage and Warping

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Large Flat Panel - Breakage and Warping

Postby meldawson » Thu Jun 14, 2018 5:11 am

Project Information:
1) Description of the Problem:
A) I used a full fuse firing cycle on a rectangular glass panel consisting of 2 layers of 3mm sheets which were not fired previously. The glass panel size is 1000mm x 500mm x 6mm. The base Tekta glass panel seems to have broken during the earlier part of the firing cycle and then it fused together again later. The break lines are also visible on the top layer of Gray and Orange sections down the middle of the piece.
B) The fired piece is also concave in the centre. The centre part seems to have hollowed out by about 4mm in the width and only about 2mm in the length – The first impression is that the batt expanded too much and sunk in the middle, causing the glass to break and the edges of the glass panel to curve slightly upwards. I also noted what looks like tears / rips in the thinfire paper causing something like stretch marks on the underside of the piece (refer to photos). However, on inspection of the batt before and after firing, there are no indication of fault lines or any warping at all. The batt is perfectly level and straight before and after firing.
Note that I fired the same sized panel but using multiple thin strips of translucent glass on a Tekta base as a test just prior to this firing, using the exact same kiln, batt and firing cycle and the panel came out perfectly fused.

2) Photos Attached
Cross Panel Breakage and Curving.pdf
(207.99 KiB) Downloaded 301 times

3) Glass Used
00126-0030-F-FULL Spring Green Opal 08/21/09 Top Layer
00124-0030-F-FULL Red Opal 04/17/14 Top Layer
00136-0030-F-FULL Deco Gray Opal 06/21/17 Top Layer
00025-0030-F-FULL Tangerine Orange Opal 03/31/17 Top Layer
00224-0030-F-FULL Deep Red Opal 03/17/17 Top Layer
00222-0030-F-FULL Avocado Green Opal 01/13/17 Top Layer
01100-0380-F-4040 Tekta Clear 03/05/18 Bottom Layer / Base

4) Design
The piece is rectangular with dimensions 1000mm x 500mm x 6mm. It was made with a single piece of Tekta Clear 3mm as the base, with pre-cut triangles in the 3mm Opal colours as the top layer. A small amount of Bullseye Glastac was used on the short side edges.

Kiln and Firing Information:
1) Kiln
An ECO170 clam-shell glass fusing kiln manufactured by Ultra-Furn in South Africa. Interior dimensions of 1050mm(L) x 540mm(W) x 300mm(H). 220 Volts, 3 x 16Amps, 10 kW. The controller is a programmable Orton AutoFire Express controller. The elements run across the top of the kiln lid only. Testing of the kiln shows very even heat distribution across the entire batt space.

2) Firing Cycle
Segment Ramp Rate (Celsius) Target Temperature (Celsius) Hold Time (Minutes)
1 165 650 30
2 Full 815 20
3 Full 510 90
4 47 410 0
5 67 350 0
6 0000 30

- The total firing time up to segment 5 of the above cycle was 20 hours and 47 minutes.
- My kiln is very well insulated, so I left the two round vents at the side of the clam-shell lid open during the entire firing cycle otherwise the same cycle takes 31 hours to complete. Even with the vents open, the kiln only cools naturally at 40 Degrees Celsius per hour from about 650 degrees Celsius downwards. I opened the kiln for the first time when it reached 50 degrees Celsius, which was 40 hours after starting the cycle.

Kiln Furniture and Supplies:
1) Kiln Shelf
The kiln shelf is an Extruded batt imported from Brazil and sized at 1000mm x 520mm x 25mm. As far as I know it is made from Mullite Cordierite.

2) Shelf support and separator
The batt was treated with Bullseye shelf primer, but I also used a sheet of Bullseye Thinfire paper directly below the Tekta base layer.

3) Shelf Position and Support
I levelled the batt on the kiln floor using an even layer of 5mm thick high quality fine Chromite Ore (less than 1% Silicone particles). There is between 5mm and 10mm spacing around all sides of the batt and the kiln wall. The top part of the batt is about level with the top of the kiln bed side walls as seen in the photos.

4) Additional kiln furniture
The edge where the break in the Gray glass is most visible was placed against a strip of 2mm thick fibre blanket and Bullseye dams along the entire side of the piece in order to get a sharper edge to eventually place the panel flush alongside the next panel in the series.

- I keep thinking it must be my batt or the bed of chrome ore causing too much expansion and / or warping of the batt during firing, but surely the batt will not return to a perfect flat surface after firing if this was the case.
- Or maybe it has something to do with the slow rate that my kiln cools off at, or the vents that I leave open.
- Honestly, I am at a complete loss on what to do to prevent this from happening again.
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Joined: Tue Dec 06, 2016 11:53 pm

Re: Large Flat Panel - Breakage and Warping

Postby meldawson » Sun Jun 24, 2018 11:57 pm

Further Test Results on Large Panels with Tekta and Opaque colours
I performed further testing to try and identify the problem with my large pieces breaking and curving upwards after firing. I used pieces of the same opaque coloured glass as in the original piece, but I alternated the bottom layers using Tekta and normal clear glass sheets. I also included a panel with only 2 layers of Tekta at the bottom and normal clear glass at the top. All other variables were kept the same.

Results: See Photos attached
A) The 3 panels with opaque coloured glass on top were all curved upwards starting from the centre of the panels outwards.
B) The panel containing only Tekta and Clear glass remained completely flat.
A) The problem must be related to the opaque coloured glass used.
B) I will perform further testing to try and determine the following:
a. Is the problem limited to the specific Orange and Grays used in the largest pieces or will it happen with any opaque colours?
b. Will the problem occur again if I place the opaque coloured panels at the bottom and the clear / tekta over the top?
c. Will the problem remain if I use 3 layers of glass? Tekta or Clear at the bottom, Opaque colours in the middle and Tekta / clear on the top again.
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Joined: Tue Dec 06, 2016 11:53 pm

Re: Large Flat Panel - Breakage and Warping

Postby Ted » Mon Sep 10, 2018 4:22 pm

Hello Mel,

Sorry for the long wait on a reply here.

Some notes here:

The crack that has healed indicates that the glass broke during heating, but before full fuse. This is likely a case of thermal shock, and could well be because the vents were open, preventing some areas of the glass from heating at the same rate as other areas, ultimately leading to a large enough difference in temperature to cause the hotter molecules to break free of the slower moving cooler particles. After reaching fusing temperatures, the cracks fused together, but the scars remain.

Another possibility is that the base piece of glass is expanding during heating all the way to the side walls of the kiln, meeting resistance, arching upwards, and breaking. It may sound far fetched, but the glass will expand .7% from room temperature until it reaches its softening point. In the case of these pieces, that means that the bottom piece needs about 7mm of space to expand or it will bow. The break does not look typical of that type of flexural buckling, but it could be a factor. Such a break would again heal at fusing temperatures.

Generally speaking, the panels are a little larger than recommended for firing in a kiln of this size. We recommend keeping the panels at least 2.54cm from the edges of the shelf all around. Further to that, we recommend keeping the edge of the shelf at least 2.54cm from the wall of the kiln all around, and the shelf elevated at least 5cm from the floor with posts in order to promote good circulation of heat and allow cooling around and under the shelf. Thermal shock and poor annealing are much more likely without this spacing. You may be able to successfully fire in the kiln shelf condition that currently exists, but to do so would require to slow down the firing both on heating and cooling.

I strongly suspect that your troubles are at least partly related to having the vents open in the kiln. It is remarkable what a difference this can make to the uniformity of temperature therein, and especially during the annealing cycle, the objective is to first remove the strain that has developed during cooling by getting the entire body of glass to be one temperature at the anneal soak temperature, and then cool in such a manner that a temperature difference of no more than 5°C develops throughout the body of glass until the strain point has been reached. There is just no way that such uniformity exists with the vents open.

So, why does it seem to work when the panel is exclusively clear, but not when there are opalescent colors in play? You can get away with an awful lot in terms of heating and cooling when you are dealing with a homogeneous body of glass. As soon as you begin to mix colors and types of glasses, or get considerably thicker, things need to be more carefully controlled. That means improving circulation in the kiln, closing the vents, ideally scaling the pieces down to better fit the kiln, and probably firing a little differently. Your idea of sandwiching the color between two layers of clear would probably help reduce the warping because it would create a more or less homogeneous outer skin and a more symmetrically designed panel (great for putting the piece in an architectural condition).

This is how I would fire a similar piece or pieces in your kiln given the current shelf condition, size of the piece, and assuming that the kiln is actually firing uniformly*:

Rate Temp Time
50 593 :30
222 660 1:00
Full 810 :20
Full 482 2:00
28 371 :00
Off room

Incidentally, this is how I would fire this piece in our kilns, assuming that everything were sized as I have suggested in my notes above:

Rate Temp Time
222 660 1:00
333 810 :10
Full 482 1:00
56 371 :00
Off room

As for the issue of the marks on the back from the ThinFire, those are typical of works of this scale. The ThinFire cracks open over spans this large. You can get a good, lucid surface by firing directly on fiber paper, onto which you have sifted dry shelf primer and then then brushed it into the weave and brushing excess off to one side.

*Here's some suggested reading that you might find of interest if you have not had a chance to see it previously:

http://www.bullseyeglass.com/images/sto ... tes_01.pdf
http://www.bullseyeglass.com/images/sto ... _paper.pdf

I hope that this is helpful.

My best,

Posts: 46
Joined: Mon Aug 25, 2008 1:59 pm

Re: Large Flat Panel - Breakage and Warping

Postby Ted » Tue Sep 11, 2018 9:11 am


Do you know who manufactured your extruded shelf that you brought in from Brazil? I know that there is a Brittish company called IPS that makes a similar shelf, but the maximum size is
762mm x 1,524mm. I mention it because I'm keen to learn if anyone makes something larger.

My best,

Posts: 46
Joined: Mon Aug 25, 2008 1:59 pm

Re: Large Flat Panel - Breakage and Warping

Postby amylwestover » Sun Oct 07, 2018 12:46 pm

Hello Mel and Ted,

This is fascinating to read as I have been having a very similar issue on similar size glass panels, 20" x 33". The panels are made of a single layer of 3mm Tekta on the bottom and a single layer of opaque glass on top which is also 20" x 33". Each panel is a layer of Tekta on the bottom and seamless sheet of an opaque color on top. I am using many different colors, artichoke, grey green, cream, marzipan, almond ect. I am experiencing warpage on the outside edges creating a concave nature to the finished piece. Reading through this thread it seems to me that the main warpage issue that carries through Mel's results and mine is that the Tekta is on the bottom.

Mel, I would be so interested to hear what else you have done? Did you do any further adjusting to your firing schedule to rid the panels of the warpage while keeping the opaque glass as the top layer? Or have you decided to fire your panels with the Tekta on top? Two different looks can be achieved with firing the clear as top or bottom layer and I am hoping to keep the look of having the opaque on top.

I may also be having issues with my type of kiln and the shelving system but before I go into any of that, I thought I would check in about the opaque color on top and if you indeed found that to be the main culprit of your warpage issue.

Any additional information to your outcome would be greatly appreciated!

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Joined: Sat Jan 30, 2016 2:17 pm

Re: Large Flat Panel - Breakage and Warping

Postby meldawson » Fri Nov 09, 2018 3:25 am

Hi Amy, we were very fortunate to be able to visit the Bullseye Resource Centre in Portland during October and I had some further discussions with Ted on the issue. Some of the other factors which may have caused it:
Factor 1 - I placed my kiln shelve on a bed of chrome ore sand which retains heat very well. This probably resulted in the bottom of the glass panels being hotter than the top side which can cause the warping. I was originally cautious to use numerous posts under the shelve, but I changed back to posts rather than sand beds after hearing this.
Factor 2 - Even though the Bullseye glass are all tested compatible, there are minor differences in viscosity between the colours and styles which can also cause the warping.

I tested using the tekta on top with the same chrome ore sand bed under the shelve and I had no warping!

Conclusion: The combination of my bottom heat and the opaque glass on top caused the warping action to be pronounced. With the Tekta on top, the 2 factors worked against each other and cancelled out the warping!
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Joined: Tue Dec 06, 2016 11:53 pm

Re: Large Flat Panel - Breakage and Warping

Postby meldawson » Fri Nov 09, 2018 3:50 am

Hi Ted,
I contacted the South African company called Kiln Contracts http://www.kilncontracts.co.za/
from whom I purchased the extruded shelve to see if they would be willing to share the information on the Brazilian company from which they imported it. I will let you know if I receive any info back from them.
Posts: 7
Joined: Tue Dec 06, 2016 11:53 pm

Re: Large Flat Panel - Breakage and Warping

Postby meldawson » Sat Nov 17, 2018 3:03 am

Hi Ted, the company in Brazil where my extruded kiln shelf came from is called Estiva Refratarios http://estivarefratarios.com.br/en/empresa/
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Joined: Tue Dec 06, 2016 11:53 pm

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