relationship between thickness and brittleness - WarmGlass.com

relationship between thickness and brittleness

This forum is for questions from newcomers to kiln-forming.

Moderator: Tony Smith

Post Reply
BadExampleMan
Posts: 28
Joined: Fri Nov 12, 2010 9:00 am
Location: Tel Aviv, Israel

relationship between thickness and brittleness

Postby BadExampleMan » Tue Mar 18, 2014 3:57 am

Newbie question: all other things being equal (using Bullseye glass) is a thicker piece more likely or less likely to crack during a slump? I'm trying to think it through and can see an argument for either way.

rosanna gusler
Posts: 730
Joined: Mon Mar 10, 2003 2:22 pm
Location: wanchese north carolina
Contact:

Re: relationship between thickness and brittleness

Postby rosanna gusler » Tue Mar 18, 2014 9:56 am

it all depends on your schedule and set up. as a rule i think that the thicker the piece, the more conservative the heat up needs to be to achieve thermal evenivity. r.
artist, owner of wanchese art studio, marine finisher

DonMcClennen
Posts: 367
Joined: Sun Mar 09, 2003 9:09 pm
Location: Ontario

Re: relationship between thickness and brittleness

Postby DonMcClennen » Tue Mar 18, 2014 12:08 pm

"Cracking during a slump" ?? have not had that problem in the last 20yrs. Low and Slow and as Rosanna said adjust for thickness. I think many or even most of the newbie problems are the result of too fast and too hot! You didn't elaborate on you schedule or the thicknesses you are using so am unable to offer specific help.
"The Glassman"

Bert Weiss
Posts: 2337
Joined: Tue Mar 11, 2003 12:06 am
Location: Chatham NH
Contact:

Re: relationship between thickness and brittleness

Postby Bert Weiss » Tue Mar 18, 2014 1:27 pm

The thicker the piece of glass the more difficult it is to heat evenly. You must not get the surface too much hotter than the core. So, slow down...
Bert

Bert Weiss Art Glass*
http://www.customartglass.com
Furniture Lighting Sculpture Tableware
Architectural Commissions

Morganica
Posts: 1079
Joined: Mon May 19, 2003 6:19 pm
Location: Portland, OR
Contact:

Re: relationship between thickness and brittleness

Postby Morganica » Tue Mar 18, 2014 2:20 pm

"All things being equal" is a pretty sweeping assumption, but I'd say the thicker it is, the more likely you are to have cracking. There's simply more chance for a big difference between temperatures in the core of the piece and the surfaces.

I don't know that I'd characterize it as a brittleness problem, though.
Cynthia Morgan
Marketeer, Webbist, Glassist
http://www.morganica.com/bloggery
http://www.cynthiamorgan.com

"I wrote, therefore I was." (me)

BadExampleMan
Posts: 28
Joined: Fri Nov 12, 2010 9:00 am
Location: Tel Aviv, Israel

Re: relationship between thickness and brittleness

Postby BadExampleMan » Tue Mar 18, 2014 3:41 pm

Yeah, I know I didn't give any real details. I had two pieces that were virtually identical except one was 6mm thick and the other was 7.5mm; fired identically the thicker one cracked apart during a drape, and judging from the positions of the pieces it broke during the ramp up to process temp.

(Once I realized it had broken I aborted the program and opened the kiln. When the pieces were cool enough to handle I put them on a table, where they promptly exploded. First time that's happened to me, it was a little exciting.)

Anyway, thank you for the input.

Lauri Levanto
Posts: 270
Joined: Tue Mar 11, 2003 7:33 am
Location: Halikko, Finland

Re: relationship between thickness and brittleness

Postby Lauri Levanto » Wed Mar 19, 2014 6:41 pm

Somewhere I saw a value that the core and surface (or any two points in a piece) must be kept wthin 20C 36 F below the transition range. In annealing the delta is 5 C. There we are worried about permanent frozen in stress. In warming up we are worried about temporarily tolerable stress.

Above transition range the glass is supposed to 'give' and no stress may accumulate.

In draping -or pate de verre over acore it may tear apart, not break.

My 0.02€
-lauri

Bert Weiss
Posts: 2337
Joined: Tue Mar 11, 2003 12:06 am
Location: Chatham NH
Contact:

Re: relationship between thickness and brittleness

Postby Bert Weiss » Thu Mar 20, 2014 9:51 am

Morganica wrote:"All things being equal" is a pretty sweeping assumption, but I'd say the thicker it is, the more likely you are to have cracking. There's simply more chance for a big difference between temperatures in the core of the piece and the surfaces.

I don't know that I'd characterize it as a brittleness problem, though.
I agree with Cynthia that there is a semantic issue. Brittle is not the best word. Stresses are cumulative. So if a piece has existing stress from a poor anneal or compatibility issues, and you add thermal shock stress, it will break. A well annealed more compatible piece might be able to withstand a bit more shock before it destructs.
Bert



Bert Weiss Art Glass*

http://www.customartglass.com

Furniture Lighting Sculpture Tableware

Architectural Commissions


Post Reply

Return to “Newcomer Forum”

Who is online

Users browsing this forum: No registered users and 3 guests

Warm Glass

2575 Old Glory Road, Suite 700
Suite 700
Clemmons, NC 27012
Phone: (336) 712 8003
Email: wg@warmglass.com