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Making A Extremely Low Shrink Clay?


Spirit

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Hi, I make instruments known as ocarinas. As for any instrument, ocarinas must be tuned. The smaller the chamber size, the higher pitched the instrument and vice versa. The problem with this is that as the clay shrinks during the drying and firing process, the pitch is significantly raised. I would like to reduce my shrinkage as much as possible to combat this issue. Iv'e read online where some people say mixing talc into your clay body will reduce shrinkage. Also I've seen where people suggest adding a grog to the clay.

 

Will both of these methods work?

 

As of now, I am using an earthenware clay with no grog. I fire to cone 06 and am having a shrinkage of around 5-6%. I know this is already a very low shrinkage but I would still like to reduce further if possible. I can fire to lower temperatures if need be since I am only doing saggar smoke firings.

 

Any help would be greatly appreciated!

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Paper clay might help here, too. You should be able to find how to make paper clay elsewhere in these forums.

Have you thought about increasing the initial size of your ocarinas by 5-6% to compensate for the shrinkage?

Do you have any pix to show what you are doing?

 

JohnnyK

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Jerry Rothman developed a zero shrinkage clay body in the mid to late 1970s for his sculptural work.  His point of view, as told to me by one of his students, was that there was not a rational reason that clay bodies should shrink.  Most of his sculptural work was done at cone 3 or less.  Aardvark Clay sells a Rothman clay that has a very low wet to fired shrinkage.   I have used it  and it works OK but is a bit "short."  I like it for sculpture and tiles.  Fires white.

You might talk to Aardvark and see what they can recommend.

 

Meanwhile, you should make some test objects of various sizes and shapes with your current clay to get empirical data on shrinkage.  You accurately measure at various stages of construction and again after firing and correlate the differences with the original dimensions for each shape.  It is a sort of "coefficient of shrinkage" table.  For my work I make shrinkage bars to estimate the wet to fired shrinkage coefficient.  You might want to make cylinders and spheres to more closely have data similar to the shapes you are going after.

 

LT

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Thanks for the replies. As of now, I do my best to compensate for the shrinkage by increasing the volume by 5 to 6 percent but it becomes increasingly frustrating to try to tune when the instrument is shrinking on you. Added to this is that if tuning is not done at the exact same time (in terms of stage of the clay) between ocarinas, then there will be varying amounts of shrinkage left as the water evaporates from the clay. This makes it difficult to get a specific point to tune to in order to compensate for the increase in pitch. 

 

I have pictures up online at Spiritwindocarina.com if you want to check out the shape. I just use the clay slab mould method to make the instruments. I've been doing it for quite a while now however as I move forward I would like to explore methods of making the process easier on myself. 

 

I'll check out the paper clay as well as the flameware and Rothman clay.

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Grog and paper will both give you a different problem with carving the critical parts. The lady at the weekly market I participate in uses B-mix or any other simple smooth body. She wasn't too picky about the clay.

 

From my own observations, clay shrinks very little from leather hard to low fire. I predict this is when she carves in the finer aspects and tunes them. You should have no trouble consistently drying to the stage.

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Just throwing out an idea here- Could you get close with the clay you're already using, and fill in a bit of the chamber with epoxy or something like that to fine tune the pitch after it's fired? I would think that with just a handful of tries you could get a master mold that is super close to where you need to be. Or have you though about making the chamber in such a way that it's adjustable, like an end that slides over or inside of the main body and can be glued in just the right spot to achieve the perfect pitch?

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From leather to fired, the tuning of my ocarinas increases about 40 cents. On a tuner, this is about half a note which is very significant. It also leaves a lot of variability within range. A smaller range would certainly reduce the variance I would think. One thing I must make clear is that I am making very precise ocarinas. They are tuned to concert pitch meaning they are tuned to fit into ensembles without sound out of tune. To do this, you must be very precise. Most makers at markets and fairs that make ocarinas are not worried about this level of precision so a little bit of variability between their instruments isn't a concern.  

 

I was thinking the same thing in regards to the paper clay and the grog. This could cause problems on the finer detailed points such as the labium edge (whistle) or the windway. 

 

Neil- This is a good idea and I have heard of some players who will actually adjust the tuning of their instruments by adding candle wax to some portions. While this works, it is somewhat of a last resort and if possible I would like the find a clay that would make this step unnecessary

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In speaking with a fellow maker from the UK, he let me know that the clay he uses is earthenware found from this website. 

 

http://www.valentineclays.co.uk/product/standard-white-earthenware

 

As you can see, the firing temperature is very low for most clays, even earthenwares. In terms of cones, I believe this is in the lower 020 range.  That is at least from what I have seen. He said his clay shrinkage is almost negligible his guess being around 1 percent. 

 

Has anyone hear used or run across a clay similar to this? I would purchase this clay however it is made in the UK and the shipping would be massive obviously. 

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