Planning For Shrinkage

[jordanro]

I am wanting to try and make sets of measuring spoons. I figured I would use a manufactured set as size references when creating them, but I am not sure how to account for shrinkage in the kiln. Never having made something that needs to have an exact (or close) size I am wondering if its that big of a deal and something that needs to be taken into account or if I should just say 'whatever, I'm sure they will be close enough'. Anyone have any thoughts, ideas, tips, from experience??

[Marcia Selsor]

you need to do a shrinkage test on your clay and fire it to whatever your high temperature is that you use.

Make a test tile and mark off 10 cm and run it through the firing. Check how much it shrank. Then you can trace the shapes, blow then up by adding the per centavo on a copy machine. I would recommend testing the final spoon with a commercial one and make adjustment before mass producing.

Marcia

[neilestrick]

Most stoneware clays shrink around 12%. Most porcelains shrink at 15-16%. So a finished stoneware piece will be 88% of the original size after shrinkage. Take the size you want the finished piece to be and divide it by 0.88, and that will give you the starting size. So a finished 5" long spoon, 5 divided by 0.88 = 5.68".

[Joe_L]

Most stoneware clays shrink around 12%. Most porcelains shrink at 15-16%. So a finished stoneware piece will be 88% of the original size after shrinkage. Take the size you want the finished piece to be and divide it by 0.88, and that will give you the starting size. So a finished 5" long spoon, 5 divided by 0.88 = 5.68".

Is shrinkage always even in all dimensions? For example does a cylinder shrink equally in radius and length?  Common sense tells me it would, but I'm learning that pottery is always unintuitive!

Joe

[neilestrick]

In theory, yes. In practice, close enough. Shrinkage can be slightly different in different directions due to clay particle orientation and compression and such, but for all intents and purposes it's equal in all directions. It's never going to be exact anyway, as moisture contents can vary in the moist clay. I always tell my customers that I'll get within 1/4", but better than that is luck.

[jordanro]

Thanks guys!

[jrgpots]

Let me throw a wrench in the calculations.

1. As mentioned the length will shrink by .88.

2. But, the area will shrink 0.88 x 0.88 or 0.7744.

3. A Volume will shrink 0.88 x 0.88 x 0.88 or 0.6815.

 

What does this mean?

1. It means that the volume of 1 tsp will shrink to 0.6815 of its volume after firing.

2. If you want to make a tsp volume after firing, you will need devide your prefired volume by 0.6815.

 

Now let's assume you want to make measuring spoons in a half a sphere shape. This means the diameter of the greenware spoon needs to be 1.136 larger.

 

So find the diameter of you measuring tsp (assuming a sphere shape spoon) and devide by 0.6815. This will give you the diameter of you greenware spoon.

 

This is assuming an average shrinkage of 12%.

If you clay shrinks 13% then you would need to devide by 0.87 x 0.87 x 0.87 or 0.6585.

If you clay shrinks 15%, then you would devide by 0.85 x 0.85 x 0.85 or 0.6141.

 

This is because the volume reflects the cubed value of the shrinkage .....(4/3 pi r cubed).

 

Everyone got it?

 

Jed

[jrgpots]

Just for fun, I calculated the needed volumes (in mls) of greenware mug sizes to make 8, 12, and 16 oz fired mugs. Mind you, I calculate these based on my math and have limited experience on the wheel.

 

8 oz fired cup.... 12% shrinkage.....347.18 ml greenware.

15% shrinkage.....385.28 ml greenware.

 

12 oz fired mug.... 12% shrinkage ....520.75 ml greenware.

15% shrinkage.....577.90 ml greenware.

 

16 oz fired mug.... 12% shrinkage.....694.35 ml greenware.

15% shrinkage.....770.56 ml greenware.

 

 

Mark, Marcia, John, Neil, and others with the experience do these values sound right?

[Babs]

Just for fun, I calculated the needed volumes of greenware mug sizes to make 8, 12, and 16 oz fired mugs. Mind you, I calculate these based on my math and have limited experience on the wheel.

 

8 oz fired cup.... 12% shrinkage.....347.18 ml greenware.

15% shrinkage.....385.28 ml greenware.

 

12 oz fired mug.... 12% shrinkage ....520.75 ml greenware.

15% shrinkage.....577.90 ml greenware.

 

16 oz fired mug.... 12% shrinkage.....694.35 ml greenware.

15% shrinkage.....770.56 ml greenware.

 

 

Mark, Marcia, John, Neil, and others with the experience do these values sound right?

For fun???Go throw some pots!

YOU have now put this poor person off the measuring spoons.

They had to standardise cups used in baking because Aunty's famous secret recipe did not work away from Aunty's favourite teacup!!

Now do this for every clay you know to shrinkage of,and publish the paper.

You may have to revisit as the clay recipes change.

Will this be the same on the Moon?

[jrgpots]

It would be really hard to work with clay on the moon  since it would loose all of its water content even before the first firing.    But what a great question...I understand that the ryolithe on the moon is very abrasive.  I would hate to throw with that type of clay too.   Some of you guys who like to "throw dry" might even struggle.  I wonder if you had a pressure controlled environment with O2 on the moon, would it be easier to pull a cylinder in the decreased gravity?  would the clay tend to fly off the wheel more?..... Babs, you started a whole new set of questions someone could ponder over for years... .

 

 

Jed

[Pugaboo]

.They had to standardise cups used in baking because Aunty's famous secret recipe did not work away from Aunty's favourite teacup!!

 

This is soooooo true, I inherited a bunch of recipes from my grandmother and great grand mother. I spent quite awhile coming up with new versions since the recipes said stuff like, add a scoop of this, a spoonful of that and a shy cup of the other thing. Took me awhile to figure out what a shy cup was! It's a cup that isn't quite filled all the way and not packed down. Don't even get me started on some of the ingredients had to google them and get the new names for old food stuffs. Add to that the odd Russian or German word thrown in to the mix just to keep it interesting. So standardization can be a very good thing when it comes to recipes!

 

T

[Mark C.]

I avoid all but lid measurements these days-my clay is about 12%

I stopped trying to make Grama's dish fit her sliver server set years ago -I leave that to folks starting out looking for heartbreak and making things 3 times.

I do know pots shrink different in different dimensions -meaning height and width. Measuring spoons ? not a thrown item I bet . They make great metal ones that are pretty close to stated values.

The only way to get this job right is make several sizes via testing as you go.

[Pieter Mostert]

So find the diameter of you measuring tsp (assuming a sphere shape spoon) and devide by 0.6815. This will give you the diameter of you greenware spoon.

 

You mean volume, not diameter, right?

[Marcia Selsor]

jrgpots,

ml and not pints, ?

Marcia

[jrgpots]

 

So find the diameter of you measuring tsp (assuming a sphere shape spoon) and divide by 0.6815. This will give you the diameter of you greenware spoon.

 

You mean volume, not diameter, right?

The volume of a sphere is 4/3 x pi x r3. So you can devide the volume by 0.6815...True.

 

And, since the volume of a sphere equation has only 1 measurement of distance, the radius, you can divide the radius by 0.6815 and get to the same place. The radius may be hard to measure, so measure the diameter instead. It is easier.

 

So the Answer to your question is: you can get the right adjustment by dividing the volume or the diameter. It will end up the same. This is unique to the sphere.

 

If you are making a cylinder, divide the volume by 0.8615.

OR

You could divide the height by 0.88 and the diameter by

0.7744, which is 0.88 x 0.88. This is because volume of a cylinder is..... pi x r squared x height.

 

As a side note, this is why mugs proportions look different after firing. The fired height will be 0.88 of its prevoius height while the diameter will be 0.7744 of its diameter.

 

Jed

[jrgpots]

jrgpots,ml and not pints, ?Marcia

Yes, I find it easier to measure mls. Most volumetric cylinders are measured in mls.

 

Thank you for the clarification. I changed my initial post to clarify the unit of measure.

 

 

Jed

[Pieter Mostert]

Jed, dividing the radius by 0.6185 isn't the same as dividing the volume by 0.6185. You'd need to divide the radius by 0.88 to get the same effect. For any 3-dimensional object, not just a sphere, dividing each length by 0.88 is the same as dividing the volume by 0.6185.

 

So for a cylinder, you'd divide both the height and diameter of the base by 0.88 to have the same effect as dividing the volume by 0.6185. Maybe you meant that you should divide the height by 0.88 and the surface area of the base by 0.7744.

[jrgpots]

Can we agree that:

1.  for a linear measurement (for shrinkage of 12%) ...divide the length by 0.88;

2. for an area (for shrinkage of 12%)....divide the are by 0.7744, which is 0.88 x 0.88;

3. for any volume (for shrinkage of 12%)...divide by 0.6185, which is 0.88 x 0.88 x 0.88?

 

 

Jed

[Pieter Mostert]

Can we agree that:

1.  for a linear measurement (for shrinkage of 12%) ...divide the length by 0.88;

2. for an area (for shrinkage of 12%)....divide the are by 0.7744, which is 0.88 x 0.88;

3. for any volume (for shrinkage of 12%)...divide by 0.6185, which is 0.88 x 0.88 x 0.88?

 

 

Jed

Sure.