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Shrinkage isn't consistent

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So I've got some strange things happening. I had a big order coming up for beer mugs, so I did a shrinkage test in a kiln that was mostly empty. (Probably 20 pieces) it's the big oval skutt kiln so it can fit around 150 mugs. I did a slow cool at 100 degrees an hour down to 1900. 

So my pieces came out and they were slightly smaller (>1 oz) than what I wanted so I threw them 1/8" taller. Wet size 5 3/8" H x 4 1/4" W. Loaded the kiln with around 120 pieces and shortened the cool down to 100° an hour until 2000°. Witness cones tell me it was slightly cooler, I shortened the cool down because it seemed to cause the kiln to achieve slightly over cone 6.  

So I take out the mugs, and they shrunk more this firing. How is that possible? I used a throwing gauge to make sure they were uniform. They shrunk more in width this time and ended up the same height as the ones that were 1/8 shorter. Any input would be great, I'm newish to pottery and live far away from any large studio. Sorry it was so long I just tried to give all the info you would need.

I'm wondering if because I pulled the clay slightly larger resulting in slightly thinner walls that it caused it to shrink more?

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Clay shrinks differently from the diameter to the height .That is its different in different dimensions . My suggestion is when you are talking to your customers you mention a variation to sizes-as its next to impossible to have them all be one exact size no matter how you measured with a  throwing stick. Do yourself a BIG favored learn to work with  some variation-as thats what makes for hand made work. Thats the reason its stands out from mass produced work-embrace this fact.If they need factory work thats slip cast within tight size dimensions thats not your job.

I could tell you all the variables but I always ask for at least a 1/4 to 1/2 inch variation depending on the forms to any order.

In my 45 years of orders thats a 1st fact to tell a customer-variation-it happens-I'm not a machine and ceramics is not a piece of metal on a lathe.

Some of the things to consider is the wetness of the clay in the 1st place as batches all vary and you have next to zero control on this. That affects shrinkage more than you think.

How much water you use to throw them-also can matter.The mix of and type some ingredients can also affect the outcome of water to clay mix(also out of your control)

You are just thinking about temp as the one factor and its is one factor but only one in a list of factors.

Changes in drying cycles-humidity can also affect outcomes-how long the clay has set since its made can affect it as well as the smaller nano particles can get wet or still be dry and suck up the water over time-another reason to use aged clay

The way clay is made also affects the water content which will affect shrinkage-filter pressed clay is always more consistent but most makers do not use this method-they blunger clay (add dry material to water mix and pug it-as time goes by it gets wetter as the smaller particles absorb water slower and over time. Filer pressing get the clay wetter than water is squeezed out then clay is pugged.

So thats how its possible. They all can mess with your best laid plans.

The real deal is the final user will not know after a few glasses of beer that its one oz more or less -the bigger issue is should they drive or ride?

There is a lot to this and its so much easier to just tell your customers that variation is what hand made itms are all about.

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the first firing had 20 mugs, and the second load 120 mugs: this difference alone plays into the equation. ( thermal mass)  Mark hit the key points, no need to rehash.

the cooling cycle affects glaze, does nothing for the clay. In cone 5-10 firings ( clay bodies matched), clay begins to vitrify at 2050F. From 2050 up to 2195F is when clay develops. A kiln loaded with 120 mugs needs to be dialed back to 108F an hour ramp climb to ensure vitrification and even distribution of heat.  

Large kilns have hot and cold spots: slowing the final ramp to peak will help level out this issue. Using cone packs to ensure you have reached peak is also advisable.  From your comments about a controlled cool: I will assume a programmable controller? If so, common for the thermocouple read out and actual kiln temp to be off ( by up to 40F) using cone packs to calibrate your temp read out is necessary in a production setting. Read TC (thermocouple) offset in your kiln manual. Peak temperature will effect shrinkage in a cone 6 firing, with porcelain being more susceptible. 


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In addition to everything above, clay particles are platelets, and the get compressed in certain directions by your fingers as you work the clay. The clay shrinks differently in different directions due to the direction of the platelets. Do differences in throwing can affect how the pieces shrink.

As you can see, there are a million variables at work here. I always specify to my customers that I will get everything within about 1/4" of the size they want. If they demand more precision than that, they have to pay for it, because it can double the time spent on the wheel.

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