Waster slab questions

[Lizardman]

I have made a largish-sculpture (prob. 20 lbs after bisque firing) and some wall pieces, The wall pieces are slabs with shallow "walls" (about 1.25") so they stand clear of the wall and the lugs/wire for hanging are hidden. I bisqued 1 wall piece flat-side-down on a thin waster slab of the same clay body and then glaze-fired it flat-side up on the same waster slab and it worked great! (Well, the waster broke, but that was the point.)

The waster slab I put under the sculpture broke into about a dozen pieces, so it isn’t re-usable. I’ll roll out another waster slab for the glaze firing, but I don’t know if I’ll should bisque it first.

Additional question: one of the waster slabs I made for a wall piece broke when I moved it. It’s about 17-18" diameter, and a piece along one edge about 1" wide broke off. The wall piece should be almost completely supported by the waster since it’s a little bigger than the piece. Do I need to make a new waster slab? Should I put the broken-off bit next to the rest of the waster?

Other, possibly relevant into: The wall pieces are bisqued at ^1 and glazed at ^04; it’s a ^5 clay. I’ve fired hundreds of wall pieces made from this clay at these temps and it works very well for what I’m making. I use full shelves. HOWEVER, these are the first I’ve built with the walls/rims — the others have all been completely flat and I fired them on grog, no waster slab. The sculpture is a ^04 clay, so bisque firing was to that temperature  and I’ll glaze fire it at ^04, too. 

Final question: do I need to put grog under the waster slab?

Thanks for your advice!

[Min]

19 minutes ago, Lizardman said:

I’ll roll out another waster slab for the glaze firing, but I don’t know if I’ll should bisque it first.

Very little shrinkage between bone dry and bisque so it's probably okay not bisque firing it if you were just going to ^04 but since you bisque to ^1 there probably will be more shrinkage so to be safe I'ld bisque it. Measure it before and after bisquing so you know for next time what the actual shrinkage is.

22 minutes ago, Lizardman said:

Should I put the broken-off bit next to the rest of the waster?

That's what I would do.

23 minutes ago, Lizardman said:

do I need to put grog under the waster slab?

Grog or sand will help the slab move on the shelf so yes I would. Some people use even coils of clay spanning the base of the work, spacing them fairly close together instead of waster slabs and grog (or sand). 

Welcome to the forum.

[Lizardman]

Thanks for the quick answer! I’ll bisque a new waster slab for the sculpture’s glaze firing — better safe than sorry. Measuring is a good idea. 

 I put grog under a waster slab for a recent piece, and when the slab broke it skidded several inches. There was grog in the element grooves all the way down that side of the kiln! I thought maybe I’d over-done it with the combo. 

The  coil method has never made sense to me because the clay is shrinking in all directions and the coils can only roll in 2 directions, but I’ll give it a try some day. 

[Mark C.]

Vacuum the grog carefully out of elemnets groves before firing

[neilestrick]

I use waster slabs for cone 6 porcelain and never bisque fire them, without problem. The slab can be in several pieces before firing and it'll still do the job.

[Lizardman]

I vacuumed very thoroughly, but thanks for the reminder!

[Jen WC]

Hi Neil,
A year late, but I hope you`ll see this. If you suggest that I start a different thread, I can do that, but as you mentioned it . . . When you say that you`re only firing your porcelain one time; do you mean that you are shaping it, drying it and glazing with clear and then firing it? Or firing it without glaze?
Thanks in advance. 
8^)
Jen

[neilestrick]

I'm only firing the waster slab one time. No need to bisque the waster. I do bisque fire my pots.