Stargazing

[ct4mom]

  Kev, What a shame it cracked. But I'm sure you will rescue it for something cool. I do love how the center sky turned out though.

[Stephen Richard]

Are you interested in sharing diagnoses as to why the break happened?

[Judy K]

I would like to hear it, if Kev does.

[Kev]

By all means Stephen...it's why we are here..to learn.

I suspect the reason was the difference in the thickness of the center as compared to the outer areas post sandblasting out the moon. Also..being a little impatient, I ramped at 250 degrees F /hr instead of the previous 200/hr..my bad! I suspect the different areas heated at different rates and thus the break.
If there are other factors at play as well, please enlighten me and everyone else reading this thread.

Thanks for the help.

Kev

[Lou Ann]

What fascinates me the most it the perfect symmetry of the crack..look at it...it's exactly the same on both sides from end to end.

Kev,

is it possible that the break is symmetrical because it is in the same spot as where the two outer pieces came together in the very first picture?

LA

[Judy K]

Just saw the cracked piece. It looks like it cracked on the way up, the edges are sharp and the frit not cooked, it also fell in the crack. Your guess of going up to fast was a good one. I wonder if there was not stress already from earlier firings making this more likely to happen on a fat up take.

[Becki]

If it had cracked on the way up the edges would be rounded. The frit is not "cooked" because it was only taken to 1375. I'm by no means an expert but it looks to me like a crack caused by poor annealing. What were all of your schedules, Kev?

[Judy K]

Yeah Becki, but the loose frit fell into the crack before it got sticky at 1200. There is a little rounding to the edge of the crack, but not much, probably from the low top temp.

[Kev]

What fascinates me the most it the perfect symmetry of the crack..look at it...it's exactly the same on both sides from end to end.

Kev,

is it possible that the break is symmetrical because it is in the same spot as where the two outer pieces came together in the very first picture?

LA

I checked that LA..it did not crack where it was joined together.

[Kev]

I'm at work and can't remember the exact schedules off hand. The schedule I posted initally is the one I used except after the initial fuse I decreased the ramp up from 1000/hr after 1250 degrees to 500/hr and used that for all the other schedules.

[Stephen Richard]

This is my summary of what I think how this piece was assembled and fired. I expect to be corrected on any misunderstandings.

The piece was 18.5" dia with a black base (spectrum/bullseye/something else?) with a clear ring and a black centre.  Opalescent frit filled the remainder.
It was initially fired at
200F-1000
50F-1250, soaked for 40mins
1000-1465, soaked for 30mins
afap-1000, soaked for 20mins
200-950F, soaked for 60mins
150-800F
300-120F, off
In subsequent firings, the 1000F/hr up ramp was changed to 500.
In the final (3rd?) firing the initial up ramp was 250F/hr

The kiln configuration and size are not known (but may be a factor if not much bigger than the size of the piece).

The most obvious thing is that 250F/hr up ramp is too fast, but 200F is acceptable, although it may be on the boundary.  We know the piece broke on the way up, because Kev looked and observed it was broken on one of the up ramps (although we do not know which one, it is implied that it was on the initial ramp up). 
Someone suggested to me that their rule of firing was that each subsequent firiring should be slower than the previous.  It has worked well for me.

I am not sure a bubble squeeze was used on every firing.  It would not be necessary, but not a problem if used.

The black absorbs heat much faster than the opalescent glass and a little faster than the clear.  The opal frit and cut pieces of glass require more heat than transparent or black.  It is not certain whether the black flowed over the edges of the central white or that the iridised black interfered with the adhesion of the white to the black, or that the white distorted over the black (unlikely, I think).

The kind of break indicates this was an annealing problem from previous firings.  It is now not possible to test this piece, both because it is black and because the break has relieved the stress. To find out if it was stressed, a similar set up, but possibly smaller, could be prepared as in this:
http://glasstips.blogspot.com/2009/08/compatibility-tests.html
People often recommend doing such a test before trying something new.  I wish I always followed that advice.

I would like to know about the distance between the piece and the sides of the kiln, and whether it is a side or top fired kiln, becasue these could be two significant factors.

One other element may be at play.  It is possible the opalescent frit began to devitrify.  This can sometimes give difficulties.
These are my speculations on the results of your firings.

[Kev]

The piece was 18.5 inches in diameter. It was constructed with spectrum glass....a black base, with a clear ring on top, a black irid disk in the enter and the rest of the surrounding area was filled with opal frit. The kiln is 24 inches wide inside with a 12.5 inch depth with the v board in place across the bottom. There are elements on the sides and on the top of the kiln.

When I sandblasted the 2nd moon and stars off, I probably blasted down to about 1/8" as I had 2 moons to go through to get rid of it. That area I filled with the fine frit and was firing it when it broke.  I only did a bubble squeeze on the intial fuse.

On the 3rd firing (the last one), if I remember correctly, I believe I peeked in though the window when it was around 1100 degrees and it seemed fine. It was when it was in the 1325-1375 range that I looked again and noted the break. At that point in the schedule it was ramping up at 500 degrees / hr.

In regards to the white, it appears with the second moon that one edge was over lapping onto the irid of the black and this is where it became distorted, like it slid across it instead of staying in a steady spot like the areas where it was on the blasted black glass.

In looking at the pieces, I see no evidence of devit at all. it's all nice and shiny. This is a good time to clarify something for me, as I find it a little confusing. Having had devit happen before, I was of the thought to ramp up quickly after the bubble squeeze to get the job done quickly and get back down afap to avoid the possibility of devit with opals. I understand the spectrum schedules are merely guidelines, but looking at them it appears that that segment is significantly faster to reach the desired fusing temp. I had actually considered with the first firing to increase by 600/hr, but not being sure, stuck with the spectrum schedule...though as an after thought I realized it is based on a 12" piece of glass as well....my bad.

Kev

PS: Thanks for helping me unravel the mystery. Hopefully us new fusers  can all learn something from this.

[Stephen Richard]

The piece was 18.5 inches in diameter. It was constructed with spectrum glass....a black base, with a clear ring on top, a black irid disk in the enter and the rest of the surrounding area was filled with opal frit. The kiln is 24 inches wide inside with a 12.5 inch depth with the v board in place across the bottom. There are elements on the sides and on the top of the kiln.

If you are using both top and side elements you can get problems as you near the side.  You have avoided this up to now.  The usual solution is to slow the schedule by a factor of 2 on the ramps up and down.  The soaks do not need to change.

When I sandblasted the 2nd moon and stars off, I probably blasted down to about 1/8" as I had 2 moons to go through to get rid of it. That area I filled with the fine frit and was firing it when it broke.  I only did a bubble squeeze on the intial fuse.

If I remember correctly, you raised the temp at 250F/hr on this third firing.  This would normally give a tendency to breaks with such a difference in thicknesses, but your next comment makes me wonder if something else is in play here.

On the 3rd firing (the last one), if I remember correctly, I believe I peeked in though the window when it was around 1100 degrees and it seemed fine. It was when it was in the 1325-1375 range that I looked again and noted the break. At that point in the schedule it was ramping up at 500 degrees / hr.

This is what causes me to wonder.  After 1000F you say you were ramping up at 50/hr.  This should not cause a breakage, as it is very slow.  After 1250 the glass is so plastic, thermal shock isn't possible.  So I am mystified.


In regards to the white, it appears with the second moon that one edge was over lapping onto the irid of the black and this is where it became distorted, like it slid across it instead of staying in a steady spot like the areas where it was on the blasted black glass.

In looking at the pieces, I see no evidence of devit at all. it's all nice and shiny.

That is good.  It was just a possibility.

This is a good time to clarify something for me, as I find it a little confusing. Having had devit happen before, I was of the thought to ramp up quickly after the bubble squeeze to get the job done quickly and get back down afap to avoid the possibility of devit with opals. I understand the spectrum schedules are merely guidelines, but looking at them it appears that that segment is significantly faster to reach the desired fusing temp. I had actually considered with the first firing to increase by 600/hr, but not being sure, stuck with the spectrum schedule...though as an after thought I realized it is based on a 12" piece of glass as well....my bad.

I take my ramp from around 540C or 677C - depending on what I am doing - up to top temperature at a rate of around 450F for 6mm glass.  I do not have problems.  Cleanliness is more important than than speed up an down to avoid devit.  Again with a side fired kiln you may have to be more careful with ramp speeds.  The area to be most careful with ramp speeds is under 540C (or the upper strain point, that temperature above which the glass is plastic enough to no longer be subject to thermal shock).

Kev

PS: Thanks for helping me unravel the mystery. Hopefully us new fusers  can all learn something from this.

[Kev]

I only ramped 50 degrees per hour from 1100 - 1250 with the first firing as a bubble squeeze, but on subsequent firings I omitted this step.

[Stephen Richard]

Ah, yes.  Thanks.  That clears that non-mystery up!  The shift from 250F to 500F at 1000F would do the thermal shock thing, even though it might not show up until a bit later.

[Kev]

Thanks for the feedback Stephen...appreciated.

In the final schedule, I ramped at 250 till 1100 then increased the ramp to 500 per hour.

Question...was the one hour hold at 950 long enough for future reference? I have not attempted pieces this large before, but wish to continue trying to as that was the reason for the large kiln to begin with.

Do you think the differences in the thickness had a role in it as well since the center was thin comparatively to the rest of the piece?

[Stephen Richard]

Thanks for the feedback Stephen...appreciated.

In the final schedule, I ramped at 250 till 1100 then increased the ramp to 500 per hour.

Question...was the one hour hold at 950 long enough for future reference? I have not attempted pieces this large before, but wish to continue trying to as that was the reason for the large kiln to begin with.
That is what I use for 10mm pieces.  The important thing is to have the glass all at the same temperature before you begin your anneal.  The soak at the annealing temperature is to allow all of the glass to be at the same temperature, so the annealing can begin.  This means that your 83C/hr (150F) annealing rate really was for 6mm glass rather than 10mm.  So I would use about 68C/hr (124F) as the annealing rate. (55C/hr - 100F - is what is used for 12mm glass).  I have never understood the Spectrum recommendation for a soak above the annealing soak.  It is not necessary when you look at what you are trying to do.  Namely, get the glass at the same temperature throughout its substance so that you can bring the glass temperature down in a slow controlled fashion.  The soak above the annealing point only extends the whole firing.
Do you think the differences in the thickness had a role in it as well since the center was thin comparatively to the rest of the piece?
Oh yes!  When I am firing glass of different thicknesses, I use a rule of thumb - not scientific! But it works.  I subtract the thinnest part from the thickest and add the difference to the thickest and use a firing schedule for that.  So 3mm from 10mm gives 7mm.  Add 7 to 10 and you need to be firing for 17mm.  So the initial rate would be about 25C/hr to about 40C above the annealing point, after which you could increase the rate to about 150C/hr to working temperature. But that is my little "rule".

I don't know if you can switch off the side elements in your kiln, but if not you will always need to be a little more careful than the published schedules.  They assume top fired kilns with raised shelves.  I would change your firing schedules by a factor of 1.5 or 2 to get safe firings.  Of course, you will do experiments and find out what the limits are better than I can tellyou.

[Kev]

I figured I had nothing to lose, so I fit it back together getting as close a fit as possible. I added more frit to the center filling in the largest gap and added some more colored frit over the piece. I will take it to a full fuse and hopefully mend the wound closed.

My schedule for this is:

150/hr - 1100
200/hr - 1465 (hold set for 30 mins- might not need that long)
afsp - 1000
200/hr - 950
hold for 90 minutes at 960
100/hr - 800
200/hr - 120

[Judy K]

I was told by a Spectrum guy once, the stop at 1000 was a place to let the glass become a little more uniform, but mostly to stop the free fall so the computer does not skip through the anneal cycle. With the wonderful new controllers maybe this idea is old, but this winter I did have my kiln free fall right through the anneal to 800F. It skipped that seg and It was a huge loss that cost me dearly.

Richard, do you have a meathod or chart for determining rates and temps, per thickness and diameter of the glass.

[Barbara]

Keeping my fingers crossed for ya.  The process to get here has been a learning experience and I hope it comes out this time.