I know I have blogged on this topic before, but since I didn’t learn my lesson the first time, I thought I would blog again to help others ensure they learn faster than I seem to have. Check out my beautiful masterpiece with a nice big crack down it!
What is annealing? It is a very important step after taking your piece to its full fusing temperature and are then cooling it down, whereby “the stress in the glass is relieved and the molecules in the glass are allowed to cool and arrange themselves into a solid, stable form.” (from the Warm Glass website)
The layout of this piece is basically a 3mm clear circle for the base. Then I added two pieces of 3mm white on the sides and black and red pieces in the center. Course frit fills in the gaps. Here is the tricky part – I added on top of the white extra pieces of black and red making this area 9mm thick and a red wavy rod spanning across the top which by itself is about 5mm thick.
Here is the key sentence from the Bullseye Annealing Chart: “If the piece is not set up in such a fashion that it can cool equally from top and bottom or is anything besides a flat slab of uniform thickness, select an annealing cycle for a piece that is twice the thickness of the thickest area of the piece.”
So, if I add up my thickest part, it is 11-12 mm thick and then double it to 24mm. The chart indicates I should have annealed this for 4 hours instead of the 2 hours I did.
This piece cracked when it was almost finished cooling (like 100 degrees F), however I had another similar piece that did not crack until I put it back in the kiln to slump and it cracked when it was heating up reaching about 400 degrees F. Stress fractures don’t always make themselves apparent right away and I have read that they can even manifest themselves with cracks often months later.
As far as I can tell from the data, it does not hurt to over anneal your glass art, so play it safe and make sure you anneal your piece for long enough to not have a similar problem.
Okay, so what to do with a rather large, 12.5″, cracked glass art? A friend suggested I display just half of it.
Perhaps because I am well aware its goal, it looks like half the art. But maybe not. What do you think?