Firing trick

[CactusPots]

Wasting some time on Youtube today and watched this video on Japanese bonsai pots.  He has just removed the pot from the stack on the left.  Notice in the picture that the entire bottom of the pot is supported with what looks like a custom cut kiln shelf.  I'm guessing because the clay is so smooth that it has very little grog or sand to support the thin flat bottoms.  Not much room for varying sizes, you need the support to match the mold.

[liambesaw]

I think it's a slab that shrinks with the piece during firing so it doesn't pull the bottom apart.  Large flat objects sometimes have a hard time shrinking without friction in a kiln, so maybe a bit of insurance.

[neilestrick]

It needs the support around the edges or else it would warp around the feet. The feet might not even be touching the shelf.

[CactusPots]

I've had enough issues with warping.  Rather than have a single piece that exactly fits only one size pot, maybe I could accomplish this with a number of individual pieces.  Take a saw blade to a mullite kiln shelf.  The pot closest to the camera in the picture looks like kiln shelf to me.  Well kiln washed shelf to be sure.

[liambesaw]

I was thinking this:

 

So maybe a slab under with shelf pieces to support the edges (since they have feet)

[neilestrick]

I think you could easily do it with some wedge shaped pieces of kiln shelf that fit between the feet. It doesn't necessarily need full support like they have, just more support than the feet.

[CactusPots]

Do you think this technique might have saved this pot?  There was no crack apparent going into the glaze fire.  The pot is about 9 1/2" side to side across the bottom.  The crack doesn't extend up the side, it's right across the foot at the joint and across the bottom.

[Mark C.]

Waster slap for sure-100% sure on that photo

Use WSO for this if its the same shrinkage rate or your same clay body or a clay body with same shrinkage rate is best 

Waster slabs are great for large stuff 

the pot you show looks like the feet can grab and stick hence the crack.A waster slab will prevent this

[CactusPots]

The clay on this pot is Special Number 2.  It's custom mixed for my local dealer Freeform Clay by Laguna.  Similar in color to Amador, less grog.  Very dark  red.  Nice clay, I'd buy more of it if it wasn't custom order.

When you say WSO do you mean Laguna's WC389?  That's a really white expensive (relatively) clay.  I'm thinking more of using old mullite kiln shelves cut up for the purpose.

I expected the waster slab to be primarily for preventing warping or sagging of the bottom.  I use grog under the feet on pots like this in the bisque fire and don't expect much shrinkage in the glaze fire.  Right?

I'd like to find a solid procedure for bigger slab pots.  I really like the slab roller and expect these to be a good seller for me.  No other defects on this pot, so I think I'm close.

[liambesaw]

The kiln shelf pieces are for support. The waster is to prevent friction cracks when fired. So putting pieces of kiln shelf would support it, but may not move enough to prevent the cracks

[CactusPots]

The video didn't show any other pots removed from the shelves.  I assumed the pieces you are calling "shelf piece" circled in yellow go all the way across the bottom of the pot the same as the "waster".  You're saying they are just on the edges?  No way to be sure from the pictures, I think.

Now I'm not understanding the waster.  Covering the entire bottom of the pot would increase the surface area and drag between the pot and the waster.  If that's the deal, it's a waster all right, cause you're wasting maybe 5 lbs of clay.  You couldn't even reuse it, because you'd need a piece of bisque to match the pot.  Got to be a better way.

If the cause of the crack in my pot is that the feet are hanging up as the pot shrinks, then I think just using a thin layer of grog would give it ball bearing to slide on and be the best solution. Y/N?  

[liambesaw]

14 minutes ago, CactusPots said:

The video didn't show any other pots removed from the shelves.  I assumed the pieces you are calling "shelf piece" circled in yellow go all the way across the bottom of the pot the same as the "waster".  You're saying they are just on the edges?  No way to be sure from the pictures, I think.

Now I'm not understanding the waster.  Covering the entire bottom of the pot would increase the surface area and drag between the pot and the waster.  If that's the deal, it's a waster all right, cause you're wasting maybe 5 lbs of clay.  You couldn't even reuse it, because you'd need a piece of bisque to match the pot.  Got to be a better way.

If the cause of the crack in my pot is that the feet are hanging up as the pot shrinks, then I think just using a thin layer of grog would give it ball bearing to slide on and be the best solution. Y/N?  

Yes. Waster slabs are used once and then tossed or turned into grog or whatever.  They shrink with the pot they're under to prevent those big gaping cracks that happen on large flat bottomed things.

I do think the shelf pieces are just bits to support the piece while it's fired.  Most of the weight is along the edge where the wall and base meet, so I'd guess supporting that area and probably a little piece under the middle of the pot too so it doesn't sag, but not so much that it causes drag while it shrinks 

[Mark C.]

Waster slabs are trash when done -they move along with the pots-Pots shrink in the glaze fire and in the bisque fire -you need to make and fire some shrinkage bars from your clay body to understand how much in both fires -(use the search function here to learn how to make and fire these as they have been written about a lot )I only use waster slabs when the work is large or problematic like say a 3 foot long fish I fired on top three shelves spanning each one.You can use colts or grog to aid the slid during shrinkage. Kiln was helps as does really flat shelves (advancers)

You will be surprised on how much the clay after bisque still shrinks when glaze fired.

I have some super high alumina mini balls that are BB sized for this as well. I was gifted these high tec items and have found the source but its only a large buy in to get any more and I did not want $500 of them. They come from the coors ceramic division -yes same as the beer in Golden Colorado 

WSO is a clay body from laguna its call number is WC 389 and thats the cone 10 version -I have not looked to see about cone 6 if thats your temp range?Its a cheap waster body and a very porous even when fired to cone 12. More like cement than clay. super weird stuff great for large work as it tough and can take torture well.

[Dick White]

Just a small technical note from my experience - I have found that there is little or no shrinkage in the bisque firing IF your work is already bone dry. The first round of shrinkage (typically about half of the total shrinkage in stoneware clays) is the loss of water from the moist clay. If your wares are put in the bisque kiln still slightly moist and candled dry, then there will appear to have been shrinkage between the time you closed the kiln and opened it after the firing, but that shrinkage actually occurred during the candling to dry, not during the firing to bisque temperature. Stoneware will shrink again as it matures and vitrifies in the glaze firing, and this is where cracking of large slab bottoms occurs, because of the friction against the immovable shelf as the slab shrinks. This is where the bisqued waster slab of the same clay body shows its worth. The waster slab and the valuable ware will shrink together, and the waster may crack from the friction with the shelf, but the ware on top of it will not. There are other techniques (e.g., alumina dusted on the shelf, ceramic balls) but those are not the subject of this topic.

[Marcia Selsor]

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

[GEP]

Am I imaging, or is there another crack here?

If there is more than one crack, and they both originate in the "corner" of a foot, then this indicates there is a structural problem there. Either the joints need to be compressed together more, or there is a lot of extra thickness there. 

If the feet are stuck to the kiln shelf while the pot is trying to shrink, that would certainly make the problem worse. So propping up the pot on wasters is still a good idea. But take some time to examine the structure in those corners too.

I hope you figure it out @CactusPots, because it's a lovely form. 

[Min]

If the reason the Japanese potter used full supports under the pots is to avoid sagging from wide bases (not being supported) I'm not so sure you would need a full sized support under the pots the size of the pot. Wide platters often have a second footring inside the first and sometimes also a clay nubbin in the middle of the base to prevent slumping. 

Since clay is going to shrink in height as well as width if the supports don't shrink and the feet are touching the shelf prior to shrinking they could well be suspended after the firing. For this reason if you use supports (made from kiln shelves or setters) they would need to be able to support the entire base. If you made disposable ones at the time you make the pots,  from the same claybody as the pots, they could be made at the same time as the pot so they shrink at the same rate and are the correct height. I could see making up a throwaway support in a hexagon shape half way between the foot and the center to support the middle from slumping to address that issue and then address the drag issue.

 

[neilestrick]

FYI, because of the reason Dick mentioned above, waster slabs do not need to be bisque fired. Nor do they need to be whole. Broken pieces will work just as well as a full slab. I roll mine very thin, about 1/16", and they often crack when the pot is placed on them. I put a coat of alumina wax on mine to keep the pots from sticking to them

[CactusPots]

If the only purpose is to keep the pot from sticking to the shelf in the glaze fire, I would think a thin layer of alumina or even grog would be more expedient and work as well as a waster.  Y/N?

In any case, if the waster doesn't slide on the shelf for some reason, you're no better off, right?  Most likely with dark red clays like this one. 

[liambesaw]

11 minutes ago, CactusPots said:

If the only purpose is to keep the pot from sticking to the shelf in the glaze fire, I would think a thin layer of alumina or even grog would be more expedient and work as well as a waster.  Y/N?

In any case, if the waster doesn't slide on the shelf for some reason, you're no better off, right?  Most likely with dark red clays like this one. 

Even if the waster cracks, it'll still shrink with the pot. So it still functions either way

[LeeU]

I was reading all this and hitting "likes" in appreciation and decided to just comment to everyone the that info is very interesting--and while I may be unlikely to need to apply it, I like having it on file (I copy stuff I might need someday and put it in a CAD Tips file). I can see how helpful all of this should be for Cactus Pots--from Neil's 1/16th of an inch slabs (!) to Mark's super weird WSO torture-friendly stuff,  to coils, balls, wash and nubbins!   

[CactusPots]

2 hours ago, liambesaw said:

Even if the waster cracks, it'll still shrink with the pot. So it still functions either way

OK, but I'd like your opinion of the option of a layer of grog on the shelf.  

Just for the record, my bottom shelf 1/2" above the soft brick floor is 1" mullite with kiln wash.  The next layer is 3/4" mullite with kiln wash.  Then usually 3 or 4 layers of cheaper silicon carbide with no kiln wash and the top shelves are Advancers.  The purpose of the mullite shelves (to my way of thinking) is to add thermal mass low in the kiln.  I'm pretty sure this crack was on a regular silicon carbide shelf.

Also, as GEP pointed out, the fault may have originated with the construction of the foot, which was installed in 2 parts.  Next time I build one of these, I will use the extruder to make a one piece foot.

[liambesaw]

1 hour ago, CactusPots said:

OK, but I'd like your opinion of the option of a layer of grog on the shelf.  

Just for the record, my bottom shelf 1/2" above the soft brick floor is 1" mullite with kiln wash.  The next layer is 3/4" mullite with kiln wash.  Then usually 3 or 4 layers of cheaper silicon carbide with no kiln wash and the top shelves are Advancers.  The purpose of the mullite shelves (to my way of thinking) is to add thermal mass low in the kiln.  I'm pretty sure this crack was on a regular silicon carbide shelf.

Also, as GEP pointed out, the fault may have originated with the construction of the foot, which was installed in 2 parts.  Next time I build one of these, I will use the extruder to make a one piece foot.

My opinion on grog, silica or alumina on the shelf instead is that those work fine for smaller stuff, but if it's something thick or heavy they don't work as great.  I fired a large jug on silica and it still broke, after taking it out, I could see that one side stayed in one place, and the other side shrunk and dragged a path through the sand.  I'm guessing that the one side got stuck so it still cracked.  Gonna fire anything over 10lbs on a waster from now on, no reason for me not to. Takes all of 5 minutes to roll out a bunch of waster slabs, took me a few days of throwing, attaching, fluting and decorating for the pot.

I don't know about grog, it is sharper and more irregular, I used silica because someone told me it was rounder in shape and more likely to allow things to slide over it.  Works fine for casseroles and other flat bottomed smaller stuff though.

[Mark C.]

Your pot in photo has many feet and any of them can grab and pull against the other feet. The waster slab moves with all the feet. If it grabs and cracks it still shrinks the same and pot and slab shrink together .They work . Yes balls and coils also work but in your case with all these feet the waster is fast and easy to make and sets up fast in terms of loading.I'm with GEP on the foot crack  in construction but the waster may still  help.

[neilestrick]

Grog, silica sand, etc on the shelf works, but it's messier, it can leave marks in your clay (especially true with porcelain), and if you glaze very close to the bottom it can get into your glaze. Waster slabs work because they crack where they need to and keep shrinking. I use them when I fire goblets. Open-bottomed pots are very prone to warping, and waster slabs keep them perfectly round, even though the slabs might be in 5 or 6 pieces by the time they're done.