Mixing Clay Bodies To Get Desired % Shrinkage

[jrgpots]

I just got an old (new to me) Brent extruder and will be making a lot more flutes.  But I am getting tired of the calculations for 12% shrinkage rates.  So I bought some heavily grogged Dixon Sculpting clay from Aardvark that has 8% shrinkage at cone 5.  

 

1.  Can I  mix that with B mix in a 50-50 mixture to get a clay body that shrinks 10%?  

 

2.  Does anyone know how these two clay bodies might play together?

 

The Dixon is Terra cotta red color in oxidation at cone 5. B mix at cone 5 is white.

 

3.  Any guesses as to what color the mix might be?

 

I thought if anyone may have tried this I could get some useful information.  Otherwise, I will try mixing them and see what I get.

 

Jed

[oldlady]

mark will say the color will be lighter than red and darker than white.   betcha..............

[PeterH]

Would a shrinkage ruler help?

 

http://lapellaart.blogspot.co.uk/2010/11/diy-shrinkage-rule.html

 

http://www.bigceramicstore.com/shrink-ruler.html

[Marcia Selsor]

I was just about to suggest that. -a shrinkage ruler or a series of shrinkage tests. 

Marcia

[Biglou13]

you're not going to like my answer........

 

TEST TEST TEST

 

could be as simple as   weigh out both, weigh grog,    wedge then mess out of it..    let rest  then wedge the mess out of it again ,     then run some test bars.....

 

might as well run some  test extruded cylinders  while your at it.......

[Mark C.]

You will need too make a shrinkage bar and test this out-and yes the color will be lighter

The only way is Test Test Test as Lou says.

[glazenerd]

If you dislike Biglous' answer: you will hate mine.

You are mixing an earthenware and stoneware body together: color has nothing to do with the mechanics. Which means you are mixing different silica meshes, different levels of flux, and different levels of alumina. The alumina blend will not have much affect; but the silica and flux levels will cause problems. Shrinkage is determined more by clay content, particle size, and LOI than the kind of clays mixed together. In the clay world: 1 + 1: does not equal 2.

I will however give you a free umbrella: after all I did rain on your parade.

Nerd

[Roberta12]

I  wonder how mixing those clay bodies would affect how glazes fit?  Just curious.  Oh yeah,  Jed.....test test test.....

 

Roberta

[Min]

If you dislike Biglous' answer: you will hate mine.

You are mixing an earthenware and stoneware body together: color has nothing to do with the mechanics. Which means you are mixing different silica meshes, different levels of flux, and different levels of alumina. The alumina blend will not have much affect; but the silica and flux levels will cause problems. Shrinkage is determined more by clay content, particle size, and LOI than the kind of clays mixed together. In the clay world: 1 + 1: does not equal 2.

I will however give you a free umbrella: after all I did rain on your parade.

Nerd

 

Umm, actually I think Jed says the colour is like terracotta but it's a ^5 clay.

 (bottom of the page)

https://www.aardvarkclay.com/products.php?cat=26

[jrgpots]

As Min mentioned, both clay bodies are cone 5, so it will not be as bad as mixing earthenware and stoneware.  it will be more like adjusting shrinkage by adding grog.  The changes in shrinkage comes from the added grog content.  I hope.

 

Yes, I use a shrinkage ruler.  I  measure the % shrinkage from wet (fresh out of the extruder) to bisqued as well as from bisqued to  vitrified. Knowing the amount shrinkage at each step makes it easier to predict hole placement.

 

I want to clay with a little tooth in it for strength, but more importantly I want to get as close to 10% shrinkage as possible.  I make Native American flutes.  The diameter and length determine the fundamental pitch, key and hole placement. Just a mm difference can drastically alter the sound.  I  tune the flutes after the bisque firing and tune then flat by  the amount shrinkage will occur in the final firing. Tuning at this point is much harder

 

My issue is that since I just got a new extruder, I need to make new tube dies.  My old dies don't work in the new extruder.  So, since I have to make new dies, why not try to simplify my calculations. I want the final tube diameters to be 1" and 3/4" after firing. Everything would be easier. For example, if the shrinkage were 10%, the linear length would be .90 of original and the area would be .81 smaller (almost 0.8).  I can calculate these in my head. It makes it much easy for hole placement and tuning.  But first, I have to make new tube dies.  Is everything as clear as mud?

 

I have about 100 lbs to play with  so the "test, test, test"-ing will start tomorrow. 

 

Jed

[Bryan Johnson]

wet-wetx%shrinkage= fired    or wet X shrinkage=fired   or wet X(100-shinkage)=fired      

and

fired/(100-shrinkage)=wet   for 12%

for 12%

1"/.88=1.13636363"    .75"/.88=0.852272727"

for 10%

1"/.90=1.11111111"    .75/.9=0.824175824"