reheating thick, obelisk shape

[Eric Baker]

Hi fellow glassers,

I've just pulled out of the kiln a cast obelisk, 1.5 inches thick at the base, 0.5 inches thick at the tip, and 8 inches long (bullseye glass). It's due for a sandblasting, before I slump it into a gentle curve.

But this is the thickest solid piece I've cast so far, and was concerned about how conservative to be on the reheat for the slump.

I followed the recommendations for the anneal and cool down that Bullseye has on their thick slab charts, but wasn't sure if I have to heat up as slowly as the chart recommends for the cooling...

My full-fuse schedule (deg. F.) was the following:
250 to 1250= 1 hour
440 to 1470= 30 min
full to 1000= 20 min
100 to 900= 6 hours
12 to 800= no hold
22 to 700= no hold
72 to room temp=no hold

For a slump, heating back up at 72 deg. per hour, or 22 deg. per hour, or especially 12 deg. per hour seemed an insane amount of time in the kiln, essentially making this 48 hour kiln cycle very nearly 5 days of kiln time. If that's what's necessary, of course I'll do it. But I don't want to waste electricity, and especially kiln availability for my lack of knowledge.

So is there a certain theory pertaining to further heatwork on thick slabs, such as slumping or firepolishing; or perhaps a good rule of thumb for how fast/slow to push the glass on any given reheat?

any thoughts?

thanks,

Eric

[Kevin Midgley]

Read Stone's book

[Eric Baker]

Hi Kevin,

I do not have Stone's book, but thanks for the suggestion...

[Stephen Richard]

I'd do the re heat at less than 25C/hr. Yes 5 days seems about right.
Nest time think of casting the curve rather than curving after casting.

[Eric Baker]

Hi Stephen,

Thanks for the reply.

Do you think a reheat of 50 deg.F./hr all the way up to my slump temp of 1150 deg.F. is a safe strategy? Or do I need to slow down the rate as I approach 700 deg.F., and again through the 800 and 900 range? (Sorry for all the Fahrenheit references--I know you're familiar with Centigrade---maybe we here in America will adopt the metric system someday. Probably not until a few more stubborn generations have passed on, though...)

I'm fearful that I'll push it too quickly between the strain point and the anneal point, before I make it to the slump temps. This is a sample for a perspective client, and it's been an interesting ordeal to make this glass. I don't want to crack it now, but if I do, I'll learn from it. It all goes into my notes, and all increases the learning curve.

I guess I'm just surprised that here on this board, we have all the discussions (and arguments) about annealing and its definition, and yet there doesn't seem to be the same discussion about reheating and further heatwork approaches, strategies, and theorems. Just the standard mantra: 'slow down on the reheat'. Well, how much exactly?

Does an annealing cycle just get reversed, for follow up processes? I cooled this glass at a rate of 12 deg.F. per hour from 900 down to 800. Then increased the rate to 22 deg.F./hr as it dropped from 800 to 700.

Does it then make sense that, to reheat the glass, I've got to heat the glass at a rate of 22 deg./hr from 700 to 800, then slow down to 12 deg./hr for the next increase in temperature of 800 to 900 deg.?

Not having any experience in thicker slab casting, I realize that most of the thick glass vets here on this forum have forgotten more than I currently know. But having or not having a copy of Mr. Graham's book, doesn't solve the inexperience/ignorance problem that I have. Any body can read a book and copy a schedule. Knowing a proper schedule for reheating thick work, and then most importantly, understanding the theory and purpose behind the schedule is what I'm after.

Thanks for your suggestion, Stephen, of a specific and slow approach. That was about half-speed of what I was thinking of trying, so I'm glad you've helped me avoid disaster in this project.

warm regards,

Eric

p.s. And yes, it had occurred to me to cast the shape in its final curved shaped. But if I get this project, I'll have to make 90 similar pieces, each with similar structure but slightly different curves and bends. After a few early fumbling attempts, I found a way to make a quick form to cast the shape consistently, while at the different sizes necessary for the project. I just didn't realize that the slump process was going to be as lengthy. It's just a surprise, that's all. Definitely something to keep in mind as I work deeper and larger, which seems to be the direction of my work here lately...

[Bert Weiss]

It is impossible to make rules about reheat because there are so many variables concerning kiln construction, element placement, glass configuration, glass placement etc. You have to seat of the pants it.

One good rule is that if the glass (not the air) is above the strain point, it will not crack. You can heat as quickly or as slowly as you need to. 50dph seems to be a slow enough ramp to cover most questionable situations. I can't remember reading any posts that said, I heated at 50dph and the glass cracked. I recently resorted to that speed, for an inch thick reheat and it worked for me. My suggestion would be to ramp slowly to 900 or 950º F then speed up to what you think will get your job done without blowing out any bubbles. My personal rule for a heatup schedule for float is to go at my chosen speed up to 1000. I consider Bullseye and Spectrum to be about 80º below float, Since my 1000 is a conservative number 900 is probably good.

[Brock]

I've just pulled out of the kiln a cast obelisk, 1.5 inches thick at the base, 0.5 inches thick at the tip, and 8 inches long (bullseye glass) . . .

. . . My suggestion would be to ramp slowly to 900 or 950º F then speed up to what you think will get your job done without blowing out any bubbles . . .

Pretty unlikely that you would blow out bubbles in a solid piece of that shape and size. Say, impossible . . .

[Eric Baker]

Hi Bert,

thanks for the reply.

So then, you do not slow down between the strain point and anneal point? There aren't any 'steps' or different ramp rates for your work. You heat up at a 'safe' (based from experience, hunch, etc.) rate from room temp to above the strain point, then get about the business of accomplishing the heat work needed to craft the piece(s) in whatever fashion necessary, then anneal the piece, etc.

I like the 'seat of the pants' part of your explanation (but we won't speak of my retirement funding...)--my life is an illustration of that philosophy. However, I was just wondering if using an anneal schedule in reverse is a successful strategy, and not just overly conservative.

We're all dealing with issues of conservation (electricity, etc.), efficiency (labor), profitability (overhead, taxes), etc. I'm trying to make the best da-- glass that I can, but already know that I have altered Bullseye recommendations for their 1/4" thick glasses to my benefit. I've found for myself through experience (and through knowledge gained from this online community) that I can shave off some time in the first 3 or 4 of my casting/fusing/tacking firings by increasing my ramp down speeds when I know that the last cycle (slump or firepolish) in the kiln will be in the future, and I can anneal conservatively and properly on the last firing--thus ensuring the client gets properly safe glass.

I like knowing where the lines are drawn in the sand, so I can then stand right at the line and wonder if it can be redrawn. I don't often poke my toe over the line to 'just see what happens', but that is how discovery, success, and yes, failure too, are found. IMHO...

I love this forum. Thanks for the input. The glass goes into the kiln this afternoon, and I'll know next week what's transpired...

[Eric Baker]

Hi Brock,

thanks for chiming in.

The slump form will eliminate the possibility of blowing bubbles. And Bert didn't have any idea of my slumping intentions, as I hadn't really described the shape/mold.

I think it would be fun sometime, to play with 'boiling' really thick glass. And I'd like to experiment with rates and top temps that balance between fire-polish and the really, high-temp stuff, and then bounce back and forth, just to see if you can induce multiple 'boils' without the glass devitrifying/breaking down, etc.

(sigh) There's so much glass to cook for nothing but the purpose of: 'wow, I didn't know that would happen'. But yet, the mortgage payment is so blasted regular as to never be far from my mind...

warm regards,

Eric

[Brock]

. . . I think it would be fun sometime, to play with 'boiling' really thick glass. And I'd like to experiment with rates and top temps that balance between fire-polish and the really, high-temp stuff, and then bounce back and forth, just to see if you can induce multiple 'boils' without the glass devitrifying/breaking down, etc . . .

Pretty unlikely that the process you describe would NOT result in devit.
Good to do though, just count on some sandblasting/cold working.

[Stephen Richard]

Hi Stephen,

Thanks for the reply.

Do you think a reheat of 50 deg.F./hr all the way up to my slump temp of 1150 deg.F. is a safe strategy? Or do I need to slow down the rate as I approach 700 deg.F., and again through the 800 and 900 range? (Sorry for all the Fahrenheit references--I know you're familiar with Centigrade---maybe we here in America will adopt the metric system someday. Probably not until a few more stubborn generations have passed on, though...)

The objective is to get the glass past the upper strain point without breaking it. You are most likely to break on heat up if you do not give the glass time to be of an even heat throughout. A consistent speed of ramp is the best way of ensuring that.

I'm fearful that I'll push it too quickly between the strain point and the anneal point, before I make it to the slump temps. This is a sample for a perspective client, and it's been an interesting ordeal to make this glass. I don't want to crack it now, but if I do, I'll learn from it. It all goes into my notes, and all increases the learning curve.

The most likely place to break the glass of this shape is considerably below the strain point. This is because the thin part heats up more quickly in response to temperature changes, than the thick part does. So you have to control the heat to be slow enough for the thick parts to catch up both with the thin parts and within the centre of the thick part. Any variation of more than ca. 5C within the glass will cause a break due to temperature differentials (sometimes called Delta T).

I guess I'm just surprised that here on this board, we have all the discussions (and arguments) about annealing and its definition, and yet there doesn't seem to be the same discussion about reheating and further heatwork approaches, strategies, and theorems. Just the standard mantra: 'slow down on the reheat'. Well, how much exactly?

Ok I will suggest a theory. The re-heat of a thick piece should be no more than 2.5 the initial cool down rate in the [proper] annealing of the initial piece. This is based on some correlation of schedules in Stone's schedules, but are only a starting point for the discussion.

Does an annealing cycle just get reversed, for follow up processes? I cooled this glass at a rate of 12 deg.F. per hour from 900 down to 800. Then increased the rate to 22 deg.F./hr as it dropped from 800 to 700.
Does it then make sense that, to reheat the glass, I've got to heat the glass at a rate of 22 deg./hr from 700 to 800, then slow down to 12 deg./hr for the next increase in temperature of 800 to 900 deg.?


No. I think you have confused the purposes of the two processes. The annealing is to ensure that most of the stresses are allowed to be removed from the piece. The heat up is to be slow enough for all of the piece to get to the process temperature in one piece. Almost all of my broken pieces [in so far as I can tell] occur below 400C. There is no need to slow the rate of advance in the 427C to 482C range.


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Eric

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[Stephen Richard]

Hi Bert,

thanks for the reply.


We're all dealing with issues of conservation (electricity, etc.), efficiency (labor), profitability (overhead, taxes), etc. I'm trying to make the best da-- glass that I can, but already know that I have altered Bullseye recommendations for their 1/4" thick glasses to my benefit. I've found for myself through experience (and through knowledge gained from this online community) that I can shave off some time in the first 3 or 4 of my casting/fusing/tacking firings by increasing my ramp down speeds when I know that the last cycle (slump or firepolish) in the kiln will be in the future, and I can anneal conservatively and properly on the last firing--thus ensuring the client gets properly safe glass.

Well now you have me revising my opinion about the speed of re-heating. If you have not properly annealed the piece already, you will have to go even slower in your rate of advance to avoid breakage. My suggestion of a rate of advance was based on the proposition that you had properly [according to Bullseye schedules for thick glass] annealed the piece. Since you have not, you are going to get bit in your trouser backside sometime soon. The cool down from the annealing soak does not use much electricity - it uses mostly time. If you have a controller that can record you electricity usage, you can check how much was used when the annealing soak started, when it finished, and when the whole firing finished. I can win the bet on where the most electricity is used, if you want to spend some more money .

Re-firing under annealed glass is a very delicate operation. In your case it will add much more to your firing time on heat up than used in a fully annealed first and each subsequent firing. Annealing is not something that can be skimped upon.

So, my recommendation for heat up on your piece for slumping is now halved.

I like knowing where the lines are drawn in the sand, so I can then stand right at the line and wonder if it can be redrawn. I don't often poke my toe over the line to 'just see what happens', but that is how discovery, success, and yes, failure too, are found. IMHO...

OK maybe I should have let you find out for yourself where the line is. My line in the glass [and that is unchangeable] is that every time you fire above the annealing point you have to anneal again as though it had never been done before.

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