flat porcelain tiles firing tips - alumina? silica? wasters with kiln wash? tips welcome.

[kristinanoel]

Hi potters - 

I'm firing some large flat porcelain discs, about 20 inch diameter, 3/8" thick. 
They've dried directly on the kiln shelves to reduce excessive handling and they get through bisque just fine. 
These are for an art piece, so some warping is fine, but cracking is not. 

I'm concerned about cracking in the glaze fire for two reasons:

• shrinking and dragging on the shelves 
• cooling temperatures

In reading the forum on related forms here, i've gleaned the following tips, wondering if anyone has additional insight on any of them, or other things I can test? 

• waster slabs - these are problematic with porcelain, as it will fuse to the piece . one suggestion was to paint the waster with kiln wash. thoughts?
• bed of silica sand - as a way to cushion and allow the slabs to move and shrink - i haven't seen a visual of this - is it a thick layer of sand? or sprinkled on the shelf like salt? 
• alumina wax - making a mixture of a small scoop of Almunia in a few ounces of wax resist, painting the bottom. any thoughts?
• place kiln posts around the edges of the slabs, to create a heat sink and slow the cooling 

Any other ideas? 

Thanks in advance!

[Kelly in AK]

Wow. 20” diameter and 3/8” thick, that’s no mean feat, in my book anyway.

Of the things you listed, two: sand on the shelves (a dusting, not a bed) and posts around it are things I’ve heard before. I agree that a “cookie” or waster slab presents some new variables and, I believe, unnecessary complications. You mentioned alumina and wax, and that reminded me of a workshop I attended with Tom Coleman and Paul Geil. They had a wonderful presentation, Tom throwing beautiful porcelain pieces and Paul explaining how to fire the kiln. Anyway, the porcelain Coleman used would fuse to the kiln shelves and pluck so the thing they did was to dust the shelves with straight alumina. If my memory is correct it was put in a  little sieve, like a sifter, and Tom just tapped it until the shelf was dusted. It’s very much like the sand idea, but alumina is more refractory and guaranteed not to stick. I’m not a fan of making extra dust anywhere, but this seems like an exceptional situation and not something you’d do day in and day out so I don’t consider it risky from a health standpoint. 

[PeterH]

A memorable quote:
 

 

[Bill Kielb]

My direct experience making these - Dragging on shelves — major issue with large discs. It’s not just shrinkage either, everything move a little bit even after fired by the amount of their COE. Something to prevent it, alumina, or silica, not a bed, just enough to smoothly slide on. (Evenly spread Salt shaker if you will). Waster slabs in theory expand and contract same as parent clay, except …… if they stick on the shelf, same issue. Cooling - lots of speculation on this, just never found a lot of validity or physics here. Kilns cool by losses through shell then radiation and conduction. Don’t open the lid, let it cool fully. 

So very flat shelf, something to slide on (alumina, silica), load towards the middle of the kiln if you are worried about cooling too quickly. Flat is less likely to drag or catch than sharp narrow foot. Eased edges - always.

If these discs have a hole in their center such as a clock face, then all bets are off as the drilled hole sets up a very common stress in these pieces and generally requires reinforcement around the hole no matter the material drilled. Google: stress concentrations around holes for a bit ot technical reasoning. 

Edited June 4 by Bill Kielb

[Min]

I would also slow the cooling down between 1100F and 1000F and fire them in the middle section of the kiln with shelves above and below.

[neilestrick]

11 hours ago, Bill Kielb said:

Waster slabs in theory expand and contract same as parent clay, except …… if they stick on the shelf, same issue.

If the waster sticks to the shelf it will crack, but it will still do the job just fine as the pieces expand and contract and move. I often use broken wasters without any problem.

Kiln wash or alumina wax on wasters works great.

[kristinanoel]

@neilestrick -

have you had success with putting kiln wash on both sides of your waster slab? so both the side touching the shelf and the side touching the ware?

Sticking to the shelf is one problem, but I'm worried those porcelain to porcelain slabs will also stick to each other. 

[Bill Kielb]

2 hours ago, neilestrick said:

If the waster sticks to the shelf it will crack

When it cracked it stuck which means when it cracked it moved independent of its normal rate. It’s often still much smoother than a shelf so most often fine and the only way but if the ware sticks to the waster and the waster cracks, same issue. A bit more rare, but same result. Alumina both sides works wonders for things that just can’t break. And yes I have seen it happen, especially clock faces.

[kristinanoel]

@Bill Kielb  

Do you swear by alumina over silica, or are they about equal?

I've got loads of silica, but no alumina.

This is absolutely worth a trip to the clay store, if need be, but if they are about the same, I'll use what i've got! 

[Bill Kielb]

I only use alumina, because I have a ton of it and it will  not stick to anything. Folks have done well with Silica though and it has better shape. All said, nobody makes silica wax. 

[LinR]

Out of curiosity why a dusting of silica and not a bed?  Lin

[neilestrick]

1 hour ago, Bill Kielb said:

When it cracked it stuck which means when it cracked it moved independent of its normal rate. It’s often still much smoother than a shelf so most often fine and the only way but if the ware sticks to the waster and the waster cracks, same issue. A bit more rare, but same result. Alumina both sides works wonders for things that just can’t break. And yes I have seen it happen, especially clock faces.

You should always have something on the waster to prevent the piece from sticking to it, whether alumina, silica, or kiln wash. Otherwise there's no reason to be using the waster at all. When the waster sticks, it's not like the entire bottom surface of the slab is fused to the shelf. It just catches in a few spots, preventing it from expanding/contracting and it cracks. But then the broken pieces can still expand and contract as needed.

[kristinanoel]

3 hours ago, Bill Kielb said:

I only use alumina, because I have a ton of it and it will  not stick to anything. Folks have done well with Silica though and it has better shape. All said, nobody makes silica wax. 

@Bill Kielb

Thanks for the tips!

let me make sure i understand - do you recommend alumina WAX, painted on both sides of a waster slab, and not alumina sprinkled on the kiln shelf and the waster slab? is this for ease of application or...?

And is there a reason people don't make silica wax? is it in some way different than alumina wax? 

[kristinanoel]

Thanks to everyone for the tips! 

I believe i'm going to try :

• alumnia wax on one, silica dust on another (I have a few spares, so i can afford a trial or three) 
• controlled cooling through quartz inversion 1100 - 1000 @Min is 100/hr rate recommended?

[kristinanoel]

2 hours ago, neilestrick said:

You should always have something on the waster to prevent the piece from sticking to it, whether alumina, silica, or kiln wash. O

@neilestrick 

this is great information and totally new to me. I have used wasters in the past for other pieces, but never put anything between the waster and the kiln-washed shelf or the waster and the ware. And as you say, the waster still serves the purpose when it breaks! 

I haven't had issues yet, do you find that's more important for porcelain?

[neilestrick]

9 minutes ago, kristinanoel said:

@neilestrick 

this is great information and totally new to me. I have used wasters in the past for other pieces, but never put anything between the waster and the kiln-washed shelf or the waster and the ware. And as you say, the waster still serves the purpose when it breaks! 

I haven't had issues yet, do you find that's more important for porcelain?

If you have kiln wash on your shelf you don't need necessarily need to put anything additional under the waster, but if you want to be 100% sure then I'd do some silica sand under it. Definitely something on top of the waster for porcelain, but good practice for stoneware, too.

[Bill Kielb]

2 hours ago, kristinanoel said:

let me make sure i understand - do you recommend alumina WAX, painted on both sides of a waster slab, and not alumina sprinkled on the kiln shelf and the waster slab? is this for ease of application or...?

I sprinkle alumina whenever in question. Alumina wax only for lids to keep the alumina in place etc… for me. Yes, anything that sticks can cause issues so common sense try not to let anything stick when in question. Waster slabs can be a nice flat place to fire, maybe flatter than most shelves. There is nothing magical about them when they stick, or your ware sticks to them. In theory they will shrink at the same rate as your ware.

Millions of pieces go through quartz inversion up and down every single day. I mention because it’s really hard to control real cooling rates in an electric kiln and they may be less than you are thinking naturally. With electronic controls, more of a drop and hold function with most kilns, but if you can do it - great! If you can’t, avoiding sudden cooling is a given, don’t open the lid.

I did add a graph of a cooldown rate for a gas kiln below because most folks don’t map it. This is for a very  leaky gas kiln that cools fairly quickly. After 1300 degrees its natural rate is well below 100 degrees per hour ( see the actual column). If you can control cool, have at it, but natural rates may be much lower than you are aware of.

 

[Min]

22 hours ago, kristinanoel said:

controlled cooling through quartz inversion 1100 - 1000 @Min is 100/hr rate recommended?

That’s what I do for refires of my large flat pieces but for something the size you are making it wouldn’t hurt to go even slower than that, say 60F. In the grand scheme of things it won’t add much to the firing cost.

edit: do you need to fire to the maximum maturity for this piece or would under firing it by 1/2 a cone or so be acceptable?

Edited June 6 by Min

[Kelly in AK]

Alumina will not develop an affinity to any materials in your clay, your shelf, or your waster slab. It’ll stay a white dry powder.

 

[oldlady]

reading this series made me think of my warped shelves.   if you are going to fire twice, mark the piece and the shelf before firing and be sure to match them the second time.

a long steel bar across my shelves tells me if they are really flat.  can i pull a piece of paper under the bar easily or not at all?