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I will be firing a piece 17"x30" and have never fired a piece this large before. The piece will have a mosaic type design on it. With it's size, I'm concerned about what thickness of base to put it on. I was planning on using 2 layers of 3mm clear. With concern on the weight, I was wondering if one layer of 3 mm clear would work because the design will cover so much of the base. Or should I use 2 3mm pieces or maybe 1 piece of a thicker clear, 4mm,5mm?

Second question is what firing schedule would you use? The biggest piece I have fired is 10x24. The artist would like as she describes 'slightly bumpy'. This is what I was thinking:
400 1200 10 hold
400 1400 15 hold
afap 960 90 hold
100 700

Any help is greatly appreciated. I was planning on firing a 10x24 piece with a mosaic design to test but wanted some help before I do that.
Carolyn

Until you decide on what the base is, no one can know whether or not the firing schedule is appropriate.

If you use two layers for the base, your piece is basically three layers thick. Fire and anneal accordingly.

FWIW, 950 is now the recommended temp for annealing System 96 (if that is the glass you are working with), and 90 minutes likely isn't a long enough anneal for a three-layer piece that is essentially tack or contour fused.

My 2 cents,
Dana W.

also, your schedule has no bubble squeeze, and with two full sheets that large, you are likely going to get bubbles.

I think perhaps the most important question (besides going much tooo quickly on heat up) (slow down I believe to 100 deg. per hour) anyways, how much room is at the ends of your kiln. Do you have bottom and top firing or side or what is the kiln shelf you are firing on. I would also slow down after the anneal . Long slow road (or follow Bullseye firing schedules but substitute for 9oo F. anneal.) anyways, good luck. A picture would really help us all. Depending on the design and colours also is very much how to know whether to add a bit of extra time or not. But better safe than sorry. Les
P.S. are you using System 96. Sorry. Yes anneal at 950 . BUT still use the Bullseye firing schedule as it is very conservative and that can only help

If I were firing this.......if you are going to use 2 sheets as a base and put "mosaic" blocks on it I would prefire the 2 sheet base with a good bubble squeeze to your base so that it is totally flat then add the design. The weight of blocks can trap bubbles in the under double layer. Personally i like NO bubbles. This is just me trying to visualize your project. Hard without pictures. If you do prefire.....on the second fire slow down on the initial heat up but if your "blocks" have space between them you will no longer need the bubble squeeze. (If they are small.....)
Let me add......i would cool slower and add to that anneal!

Laurie Spray wrote:If I were firing this.......if you are going to use 2 sheets as a base and put "mosaic" blocks on it I would prefire the 2 sheet base with a good bubble squeeze to your base so that it is totally flat then add the design. The weight of blocks can trap bubbles in the under double layer. Personally i like NO bubbles. This is just me trying to visualize your project. Hard without pictures. If you do prefire.....on the second fire slow down on the initial heat up but if your "blocks" have space between them you will no longer need the bubble squeeze. (If they are small.....)
Let me add......i would cool slower and add to that anneal!

All responses sound like really good advice especially prefiring the base. For future reference on firing schedules and understanding issues you may be introducing to a project, I highly recommend getting Firing Schedules for Glass, The Kiln Companion by Graham Stone http://www.warmglass.org/books/books/126-fsg. It is a rather tedious read and requires some easy updates due to Bullseye anneal change but exciting because of all the insights gained.