Slow Glaze Firing for Sculptural Work?

[tman123]

Hi, 

I have a piece that is about 31' high by 15' wide and probably at most 1.5 inch thick in thickest areas. 

I'm wondering if I need to do a slow glaze firing and if I do should I run my own super slow glaze program? Will firing super slow possibly mess up any glaze and is it even necessary because it's already survived the bisque. What are the benefits of a slower glaze firing for sculptural work?

Any advice would be appreciated. 

Thank you

[Min]

Hi and welcome to the forum. 

I would look at what the slow glaze program is going at during the 1000F - 1100F range, if it's more than 50F / hour then I would slow it down during that ramp. I would also slow the kiln down during cooling during the same range and not open the kiln until it's at room temp. This is something I would be looking at doing to help equalize the temp within the kiln and doing it slowing as that is a large piece and there will be substantial differences between the 1.5" thick areas, where it sits on the shelf and thinner areas.

If you aren't sure what your glazes will do in a slow cool then I would suggest running some test pieces through the firing before your large sculptural piece. I don't know what your final cone is that you are firing to but another option would be to fire it to your target cone using the slow bisque program instead of the slow glaze program then add a slow cool. Look up your make of kiln and what the preprogrammed firing programs are and compare the two.

(I'm going to delete your duplicate post in the Chemistry section)

[Hyn Patty]

Also watch out - if your piece is on a base of some kind that is thick or wider than parts above it that are slimmer, you should stilt the piece to raise the bottom off of the floor of your kiln a little bit.  Uneven cooling can result after firing if the base retains a lot more heat due to contact with the thick kiln floor and the rest of the piece cools faster.  This can cause cracking or even dunting later well after firing.  If you do what Min suggested and slow down your cooling cycle by ramping it that will also help a lot to give the piece time to slowly equalize the heat throughout.  Kind of like the long heat cycle given to blown glass pieces so that it can relieve the stresses in the worked material.   As I fire a lot of horse pieces with very slim legs that are on thicker bases this can especially be a problem with the glassier porcelain clays or bone chinas.  I find it to be less of an issue with more porous clays like earthenware as they are less thermally conductive.

[tman123]

can the stilts support the weight of the pot? will it not sag between stilts?

[tman123]

also, sorry but why does it matter to slow the firing down during that 1000-1100 degree mark?

[Min]

For a large piece I would roll out a slab of the same clay as your sculpture is made from, make it about 1/4 - 3/8" thick then cut strips of it about 3/4" wide. Dry the strips between boards (or drywall pieces or whatever). When you go to fire your sculpture lay the strips down first, gaps between them, and sit your sculpture on them. (don't need to bisque fire them but make sure they are bone dry before firing and fire slowly) What you are trying to do is raise the piece off the shelf and allow air circulation underneath the piece to help even out the temperature.

Re slowing down between 1000F - 1100F both when heating and cooling, it's at approx 1060F that quartz inversion happens. As the quartz crystals change from an alpha crystal structure to a beta one there is a volume change of 1 - 2%, this can stress the clay and cause cracking / dunting. 

 

Edited January 17 by Min

[oldlady]

thanks for the actual number, madeleine.   i watch my firings so i can close the lid totally after quartz inversion.   the lid is left resting on top of a 1/2 inch block of firebrick until it reaches 1200 degrees or stops steaming.   now i can remove that block at 1100 degrees.   (still checking the steam!)

 

[Sandi0720]

I have a Skutt kiln with the newer electronic keyboard. I do have information from one skutts tech support on how to program your kiln to a  slow cool down.  I wanted to do this so I wouldn't get stress cracks in my sculptures. You can also verify that this will work for you have a Skutt kiln. I can post all the info if anyone is interested and the name and ext. of the Skutt tech I talked too.

Edited January 28 by Sandi0720