Firing trick

[Smokey2]

23 hours ago, Marcia Selsor said:

I use coils and use them repeatedly.I extrude 1/4" coils through a steel mesh I put in my extruder. 

From leather hard to after bisque my clay shrinks 5% and another 6-7% at ^6

Marcia

Do you apply kiln was on the coils and have you noticed any slumping of the pot between the coils?

[CactusPots]

Just to be clear, the waster slab raises the feet off the shelf?  For a pot that has a large horizontal bottom,  the waster slab should have as much contact with the bottom of the pot as possible?  A red clay is going to have iron as colorant and it's also a flux, so would you use the same clay as a waster face to face without wash etc?  I don't use porcelain, but the same question would go for those that do. 

[neilestrick]

51 minutes ago, CactusPots said:

Just to be clear, the waster slab raises the feet off the shelf?  For a pot that has a large horizontal bottom,  the waster slab should have as much contact with the bottom of the pot as possible?  A red clay is going to have iron as colorant and it's also a flux, so would you use the same clay as a waster face to face without wash etc?  I don't use porcelain, but the same question would go for those that do. 

For a waster slab, the feet would sit right on the slab. It would allow the pot to move on the shelf. It would not provide additional support, though. If you want more support than the feet provide, then you should use kiln shelf pieces under the body of the pot, lifting the feet off the shelf.

[CactusPots]

Then that's not a waster slab in the video.  There are clearly cutouts where the feet are.  It looks to me like it supports the bottom only.

Here's the link to the video, he takes the pot off at 17:54 .

 

 

[neilestrick]

Correct. I think that's providing support so it doesn't warp. A big wide base like that would  definitely want to warp with those small feet. You could do a combination, where you have the shelf piece to support the base, with a thin waster slab on top of that to allow for shrinkage.

[Min]

@CactusPots, if you go to the Monday 1 May 2017 entry on this bonsai pot makers blog you can see the supports he uses and that he makes them at the same time he makes the pots so they are the correct thickness to support the pot. Just another option that would address the first issue, sagging bases. It was what I was trying to describe in my post earlier, maybe a picture makes it clearer. Second issue, warping / cracking from drag, just fire the whole thing, supports and pot on top of a waster slab. Brush some of your alumina wax between the supports and the base plus on the waster slab under the feet if you are worried about plucking. 

[CactusPots]

I don't really have any issues with saggy bottoms.  I think their issue is the combination of long spans and thin clay.  I have been steadily working back towards thicker bottoms.  I want to be able to easily pick up a green ware  planter upside down with my finger in the drain hole.

Now I have a different awareness of shrinkage/movement in the glaze fire, so I'll take that into consideration when loading.

What difference do you think the kiln shelf makes in the ability of the pot to move in shrinkage without binding?  Kiln washed mullite/Silicon carbide/advancer?  I think the only issue is that the kiln wash is never perfect and maybe a raised piece could catch.  Maybe also my penchant for stacking pot in pot can put a fair amount of weight on the bottom pot.

Really, I haven't had this problem, but I've never seen the concept of waster slabs, so good learning opportunity, thanks.