Clay Shrinkage

[JoLinDesigns]

I need to end up with a 12" tile at 1/2" thick. I read that the shrinkage rate is 12%. Is that after the final firing? And does that also pertain to the thickness? Firing to ^5. Using Laguna WC‑401

B‑Mix 5 Clay. Thank you. Linda

[neilestrick]

If the clay shrinks 12%, then the finished piece will be 88% of the original. So 12 divided by .88 equals 13.64. It shrinks in all directions, so for thickness, .5 divided by .88 equals .57.

[Isculpt]

thanks, Neil.  I'm probably not the only person grabbing my notebook to write that formula down.......Jayne

[Marcia Selsor]

I do shrinkage tests using 10 cm on a tile. Centimeters are so much easier to work with for % measurements.

 

 

Marcia

[williamt]

Agree with working in metric. 10 and percentages just go together

 

One thing. I'm not sure if it is original moisture content or some other variable (probably more than one). The percentage shrinkage you get in a test fire will be good for that batch (lot number) of clay and that percentage may differ slightly from advertised. I think the clay manufacturers should show deviation numbers. Also I've seen the same clay type, but different batches measure a little different in test, so you probably should do a shrink test for each batch/lot number you have.

 

Of course firing temp also affects the number. A cone 6 clay shrinks a tiny bit less at cone 5.

 

The numbers I'm talking seem to usually be about +/- 1% or less, so maybe that is not a concern to you.

 

And your 12% number is at cone 5 for B-Mix.

 

Lee

[neilestrick]

With a tile you could also get different directional shrinkage depending on how you make the tile. There are a lot of variables at work here, so don't count on your tile fitting perfectly into an existing hole.

[JoLinDesigns]

Characteristics

Cone: 5

Wet Color: Light Gray

Fire Color

Oxidation: Cream

Reduction: Off-White

Texture: Smooth

Penetrometer Target: 6.75

Avg. Shrinkage 2±% 12%

Avg. Water Absorption 1±% 2.3%

COE x 10-6: 5.74

 

Is what thus clay does. The bar will be made to fit the tile which is a plus! Thank you all of your help!!

Linda

[JoLinDesigns]

Marcia. I did make a tile to test with using the centimeter way. Still waiting for that to dry so I can fire it. I did one , but the old kiln wasn't firing to the right cone, so I know that wasn't an accurate read. Thank you.

[Chilly]

You also need to make all your tiles at the same time from the same "dampness" clay if rolling and cutting or pressing.  One lady in my pottery class a few years back made a set of plaques over a number of evening classes.  She used the same paper template, and clay from the same bag, but they finished up different sizes.  Looking back, we could only guess that the clay had been at different stages of wet/damp/leatherhard when they were cut out.

[schmism]

If the clay shrinks 12%, then the finished piece will be 88% of the original. So 12 divided by .88 equals 13.66. It shrinks in all directions, so for thickness, .5 divided by .88 equals .57.

 

While mathematically correct, it may be somewhat confusing to understand whats going on above.

 

If your clay shrinks by 12% you want your starting item to be 12% oversize,  so that when it shrinks it will be the right finished size.   It may be easier to think of the above as  Finished_size*1.shrinkage_amount.   or 12"*1.12 = 13.4" start size.

 

Variables that affect shrinkage mentioned above still apply.

[neilestrick]

 

If the clay shrinks 12%, then the finished piece will be 88% of the original. So 12 divided by .88 equals 13.66. It shrinks in all directions, so for thickness, .5 divided by .88 equals .57.

 

While mathematically correct, it may be somewhat confusing to understand whats going on above.

 

If your clay shrinks by 12% you want your starting item to be 12% oversize,  so that when it shrinks it will be the right finished size.   It may be easier to think of the above as  Finished_size*1.shrinkage_amount.   or 12"*1.12 = 13.4" start size.

 

Variables that affect shrinkage mentioned above still apply.

 

 

If you do it that way you'll be off. 13.4" with 12% shrinkage puts you at 11.8", over 3/16" too small. Multiplying the finished size by 12% is not the same as shrinking the wet size 12%.

 

The formula is:

X - %X = Finished size

 

So in this case:

X - .12X = 12

so .88X = 12

so X= 12/.88

so X=13.64

[schmism]

 

 

If you do it that way you'll be off. 13.4" with 12% shrinkage puts you at 11.8", over 3/16" too small. Multiplying the finished size by 12% is not the same as shrinking the wet size 12%.

 

The formula is:

X - %X = Finished size

 

So in this case:

X - .12X = 12

so .88X = 12

so X= 12/.88

so X=13.64  

 

so what happens when you take 13.64 and divide it by 1.12  (12% larger)   13.64/1.12 = 12.18  hmmmm theres that pesky 3/16 again except this time its oversize.

[JLowes]

13.64 x .88 = 12.0032, point being .88 is what the clay shrinks to, or 100% - 12% = 88%.

[Mark C.]

Variables that affect shrinkage mentioned above still apply.

Its more than just math-its how you make the tile-you will see that exact fits are very hard.

Mark