[Clayart] Large tile question

ronroy at ca.inter.net ronroy at ca.inter.net
Sun Jan 22 16:52:14 EST 2017

Most clay slabs have inconsistencies built into them. Compressed clay  
towards the center and stretched around the outside. This means the  
outside clay will shrink more during drying and firing. The result is  
warped tiles.

If you really concentrate on building the slab without building the  
inconsistencies in to it you will stand a better chance of not having  
it warp.

One way is to recompress the clay around the outside edges.

As Snail says - not trimming the outside edges until some of the  
drying has taken place is helpful - just mark the outside edges before  
shrinkage takes place and cut to size when the slab stiffens.

Wedging some grog into Bmix will help a lot. You might do a few tiles  
out of clay with different amounts of grog wedged in to see how that  
helps with warping.

I do think it is wise to make smaller tiles. The problems are much  
more manageable.


Quoting Snail Scott <claywork at flying-snail.com>:

>> On Jan 19, 2017, at 11:28 AM, Ken Chase <kchase235 at gmail.com> wrote:
>> I want to make a tile 11X17" to top a wood table. I'm using Cone 5 Bmix...
> Calculating the shrinkage is not difficult, and you can be reasonably
> accurate. Make tests and measure; the process is described by
> other Clayart respondents, and is shown in many books and websites.
> This isn?t really the tough part.
> The most difficult shape in ceramics is a large rectangular flat slab.
> Flat is tough because there are no perpendicular shapes to brace it
> and resist  bending.
> Corrugated cardboard is made from three thin pieces of paper. Think
> of bending three pieces of plain paper that all lie co-planar, then think
> about the strength conferred by just one of those pieces bent at angles
> to the main plane and well attached. A plain flat slab is the paper in this
> analogy.
> As clay dries, it shrinks, and the top surfaces of anything will dry first as
> the moisture evaporates and/or sinks. This means the top surface will
> tyr to become smaller than the bottom, and will cup upward if it can to
> make this happen. The broader the slab, the larger the potential variance
> in height as it warps. Shapes with corners are also worse than round
> ones, as corners will dry faster than the middles of edges. If one edge
> dries faster than another, you will also finding it drifting away from
> rectilinear straightness, too.
> The choice of an appropriate clay body can have a major effect on these
> issues. I understand you may wish to use B-Mix because it?s what you
> already have in the studio, or you like the look of it, but another body
> may serve you better. The high plasticity of B-Mix - such an asset when
> throwing - is a hindrance for unmodeled and/or large forms such as the
> tile you?ve described (which is both). A coarser clay body will have
> reduced shrinkage and also tend to dry more evenly, both of which will
> reduce the warping and potential distortion. If you choose to continue
> with the B-Mix, you will have to take extra care to ensure success.
> Some helpful measures:
> Don?t work too thin. If you?ve mainly been making pottery, you can find
> that your mental bias is toward thinness?fight that! 1/2? or 1 cm is
> a good and reasonable thickness for a large slab or tile.
> When you roll your slab (whether by hand or roller), roll it halfway to the
> correct dimensions, then flip it over and roll from the other side, and roll
>  both lengthwise and crosswise. (Or do this several times.) This will allow
> you to avoid incorporated a pre-disposition toward warping before you?ve
> even begun.
> Roll the clay at a nice workable moisture level, but don?t cut the precise
> dimensions until it?s stiffened a bit. This will allow greater accuracy and
> also jump over the warpiest phase of the process. Remember, though,
> that the clay needs to be at the same dampness as when you measured
> for your tests, or it won?t be accurate. If the test was made on quite damp
> clay, but you cut the real slab from stiffer stuff, you?ll end up with the
> wrong size slab at the end. Do your initial shrinkage tests on stiffer clay
> also, to allow for this.
> Dry it very, very slowly, to allow the moisture to migrate evenly throughout
> the slab. My method: Use a piece of drywall a bit larger than the slab.
> Tape the edges to eliminate gypsum crumb contamination.  Put a piece of
> thin synthetic gauze (thrift store curtain sheers are great) down to reduce
> friction and allow freer shrinkage. Put the slab on that, cover with another
> piece of gauze, then another sheet of drywall. Weight the top piece of
> drywall down. Wrap the edges of the stack  in plastic to keep air circulation
> away. Flip it over daily, or for faster drying, swap the drywall for  
> fresh dry
> pieces daily.
> If you do more than one slab at once, you can stack them all on top of
> one another with drywall (and gauze) between each layer. This will dry
> slower, and since only the top piece of drywall is getting evaporation,
> either re-stack in changing order or cover the top piece to retard its drying
> rate to something approximating those in the middle.
> When you fire, use marbles of clay to elevate the slab above the shelf so
> heat can reach it more evenly. They will also act as ball bearings to reduce
> friction and allow more even shrinkage. If you set them in a bed of sand,
> you can level the slab even if the shelf itself is warped. Fire slowly, and
> cool slowly.
> I?d do more than one slab anyway, even if your project doesn?t require it.
> Then you can try your design more than once, and use the ?pick of the
> litter? for your final tabletop. (Sell the rest as wall plaques!)
>                -Snail

Ron Roy
ronroy at ca.inter.net
Web page ronroy.net

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