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Mossyrock

Measuring For Clay Shrinkage/grout Lines On A Fireplace Surround

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I am creating a scenic tile surround for my fireplace.  The clay has a shrinkage rate of 9%.  Do I take the measurements of the fireplace and add 9% then build the surround on that scale?  What about space for the grout lines?  The tiles won't be square....they'll be cut to work with the design of the scene (hopefully).  This is the first time I've taken on a project this large and involved.  I want to make sure my measurements are correct so it all fits properly.  Thanks for any help and advice.  

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Hi Mossyrock

 

Mathematically, that won't work.

 

Say your tile needs to be 100mm wide.

Add 9% = 109mm.

Take 9% from 109 = 99.19mm

 

That's a small difference in 100mm.  If the tile is larger, the difference is worse.

200mm tile

Add 9% = 218

Remove 9% from 218 = 198.38mm

 

There is a mathematical way to calculate this, but at 6am I can't think of it.  I'll look it up and post it later.

 

Also, depending how you make your tiles, they may shrink more in one direction than the other.  

 

Only way to really know is the usual answer: test, test, test.

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OK, the maths to work out the shrinkage rate:

 

Say tile is required to be 100mm and shrinkage is 9%

 

Divide the required size by the opposite of 9%, which is 91% or 100-9 = 91

 

    100/91 = 109.89 - size to make the tile

 

Calculate that back the other way

 

    9% off 109.89 = 100     or    109.89*.9 = 100.

 

As you are making shaped tiles, this might be tricky to calculate.  Unless you draw your shapes in something like CorelDraw, where you can enlarge them by a percentage.

 

This is probably why commercial tiles are made with a dry high-talc material and press-moulded at hundreds of pounds of pressure to make a solid mass that can be fired and will not shrink at all.

 

As for the grout lines, if you make the edges of the tiles angled, so your tile starts off at the calculated size, and you remove some clay from the top  /====\, that might give you enough space for grout, assuming you want minimal grout lines, maximum tile view.  I have a lovely tool for mitering sides of slabs, It's a block of wood with a square cut out and a wire across at an angle.  Best picture I can find........https://uk.pinterest.com/pin/307300374547891911/

 

But still test, test, test.

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Hi Mossy;

 

Chilly has already done the shrink math for you; but let me give you some additional information on doing tile work. It is what I do for the most part.

 

Use cheap vinyl tile to make your patterns, and do the complete layout before you start cutting any clay.

 

CC5 tips2

 
Arrange the pattern where it butts together; shrinkage will take care of the grout lines.
 

Trifecta

 
You said "large tiles: so you also need to observe some firing precautions; Stack the tile on edge during bisq to reduce shelf drag created by large area contact.
 

Flat Firing

 
I use old tile setters to keep them standing straight, and one on top to to keep them aligned. You can lean them against shelf posts as well: many do.  You also need to exercise some caution in the bisq firing cycle: slow it down to begin with. Large tiles and objects go through greater mechanical stresses than smaller pieces. Quartz inversion occurs at 573C (1063F): and going too fast through this temp zone will split your pieces. Ramp at 160F to 1000F, and then 100F from 1000F up to 1125F: this will give your pieces plenty of time to expand. You can ramp back up to 160F after 1125F
 
When glaze firing; if the pieces are large; then you need to put silica / sand under the pieces so they can move on the shelf. Too much shelf drag will split them.

 
You can also use slats under them if they are large and heavy; but that is a judgment call on your part.
 
Nerd

 

 

 

Mossyrock, Min, Chilly and 2 others like this

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I agree with all of the above. Chilly directions are great. Well written. Nerd use of the linoleum template is excellent. My preference for the glaze firing is to extrude 1/4" coils for the tiles to set on. It allows more eat circulation under the tiles. Use lots of them and direction of the coils can very and accommodate your shapes.

 

Marcia

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Is the 9% shrinkage what you have measured or what the manufacturer says it is? If it's not from your shrink test I would do one of those for a project this large. If you haven't done this before what you do is cut or roll out a couple 10cm strips of clay the same way you will be making the real tiles. Mark of 1cm points really accurately then bisque and fire the same way you will be firing your tiles. (fire the test strips in a couple different places in your kiln). Then measure the actual fired shrinkage. 

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I have learned never to trust the shrinkage given by the manufacture . I have always tested and its usually a bit different-do as Min says and do the test.

The last note is it is always good to give yourself some leeway and thats whats grout is for. Tell your client that it will be close and that grout will make up the difference . That way there is less pressure on you.

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OK, the maths to work out the shrinkage rate:

 

Say tile is required to be 100mm and shrinkage is 9%

 

Divide the required size by the opposite of 9%, which is 91% or 100-9 = 91

 

    100/91 = 109.89 - size to make the tile

 

Calculate that back the other way

 

    9% off 109.89 = 100     or    109.89*.9 = 100.

 

As you are making shaped tiles, this might be tricky to calculate.  Unless you draw your shapes in something like CorelDraw, where you can enlarge them by a percentage.

 

This is probably why commercial tiles are made with a dry high-talc material and press-moulded at hundreds of pounds of pressure to make a solid mass that can be fired and will not shrink at all.

 

As for the grout lines, if you make the edges of the tiles angled, so your tile starts off at the calculated size, and you remove some clay from the top  /====\, that might give you enough space for grout, assuming you want minimal grout lines, maximum tile view.  I have a lovely tool for mitering sides of slabs, It's a block of wood with a square cut out and a wire across at an angle.  Best picture I can find........https://uk.pinterest.com/pin/307300374547891911/

 

But still test, test, test.

Math makes my head hurt! :)  My surround is 55 inches across the top and 35 inches tall.  Each side is 9 inches wide.  On one side I plan to carve a heron standing on a piece of driftwood and a few reeds.  The other side will be carved cattails and reeds with a dragonfly.  In the center of the top will be two intertwined heron feathers.  The background is a very light sort of paragon repeat pattern.  I thought I could measure out the entire piece, enlarged according to shrinkage rate, lay out the slabs and carve away, then cut the individual pieces so the grout lines would sort of be hidden within the design.  Can I use your calculation method for the overall size then do the carving and cutting for grout lines as planned?  

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I have learned never to trust the shrinkage given by the manufacture . I have always tested and its usually a bit different-do as Min says and do the test.

The last note is it is always good to give yourself some leeway and thats whats grout is for. Tell your client that it will be close and that grout will make up the difference . That way there is less pressure on you.

LOL, my client is me :) so I'm pretty safe there.  I just don't want to end up with huge grout lines.  I will do a shrinkage test though just to be on the safe side.  The 9% came from the manufacturer.

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