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Throwing Items of a Certain Volume/ Capacity?


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Hey guys, a curious thought occurred to me.  Is there a particular methodology for ensuring that what someone is throwing--whether it be a bowl, bottle or mug--will hold a set amount of liquid by volume?  Or, is it just trial and error, and keeping notes when you get the right dimensions?  For example, here in the US most mugs are generally about 8 oz. in maximum capacity (although I prefer a generous 12 oz. myself!).  But, what if one has a lb. of olive oil, and they want to put it into a handmade ceramic storage container instead?

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1 hour ago, Black Phillips Adept said:

But, what if one has a lb. of olive oil, and they want to put it into a handmade ceramic storage container instead?

Liam has it. Volume is usually measured in ounces in the US so I think pound is a typo. You can do this for any shape, just reduce it to cubic inches, account for shrinkage first by multiplying required cubic inches by 1+ your total shrinkage. 12 fluid oz =21.65 cubic inches

For 15% shrinkage 21.65×1.15=24.89 call it 25 cubic inches. Stick in a cylinder volume calculator using 25 in cubic inches as the volume and pick a height or diameter you like.

If your math is good, volume of a cylinder is pi x radius squared X height. Rectangles are length X width X height all in inches of course. If not, internet volume calculators are available.

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Start with the cylinder, and adjust as needed for the form you're actually making. It'll take a little trial and error but the cylinder will get you close as starting point. If you don't want to go go the trouble of firing the pieces as you figure it out, use a measuring cup to fill them with water once they're firm enough.

I would say the typical hand made mug in the US is more like 10-12oz. Commercial mugs are the same nowadays, but the old classic thick brown diner coffee mug was only 6-8oz.

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On 9/30/2020 at 7:31 AM, Bill Kielb said:

Liam has it. Volume is usually measured in ounces in the US so I think pound is a typo. You can do this for any shape, just reduce it to cubic inches, account for shrinkage first by multiplying required cubic inches by 1+ your total shrinkage. 12 fluid oz =21.65 cubic inches

For 15% shrinkage 21.65×1.15=24.89 call it 25 cubic inches. Stick in a cylinder volume calculator using 25 in cubic inches as the volume and pick a height or diameter you like.

If your math is good, volume of a cylinder is pi x radius squared X height. Rectangles are length X width X height all in inches of course. If not, internet volume calculators are available.

Maybe I'm getting confused between linear and volume shrinkage.

Shouldn't that be 21.65x1.15³ = 32.33

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I love the equations, but when it gets down to it - trial and error. There are a few unknown variables that come into play... how thick do you throw your walls? When I first started, I would throw a mug with 1 lb of clay  and after shrinkage I would get an 8 oz by volume mug. Now I get as much as a 12 oz mug because it is taller and thinner. This also depends on the type of clay I am using and how well it holds up to throwing thin. 

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2 hours ago, PeterH said:

Maybe I'm getting confused between linear and volume shrinkage.

Shouldn't that be 21.65x1.15³ = 32.33

Nope, cubing percentage? The 21.65 is a volume in cubic inches. 15% more liquid is fifteen percent more. No need to cube the percentage. Cubing this makes it 50% bigger instead of 15%.

Edited by Bill Kielb
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11 hours ago, PeterH said:

Maybe I'm getting confused between linear and volume shrinkage.

Shouldn't that be 21.65x1.15³ = 32.33

 

9 hours ago, Bill Kielb said:

Nope, cubing percentage? The 21.65 is a volume in cubic inches. 15% more liquid is fifteen percent more. No need to cube the percentage. Cubing this makes it 50% bigger instead of 15%.

Let's try that again. Can you confirm that we are talking about 15% linear shrinkage?

If so, lets say we want to finish with a cube of 1" side, volume =1x1x1 cubic inches

Using the calculator from How and Why To Make A Shrinkage Measure https://www.ceramicartsqld.org.au/index.php/component/k2/how-and-why-to-make-a-shrinkage-measure

shrinkage.jpg.0e3c3897e2fda7953918fdb4be358d25.jpg

So the volume of the wet cube is 1.18x1.18x1.18 = 1.64 cubic inches

PS The calculator uses 1.18 = 1/(1-0.15) rather than 1.15 as the shrinkage is 15% of the wet size rather than the dry size.

 

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23 minutes ago, PeterH said:

 

Let's try that again. Can you confirm that we are talking about 15% linear shrinkage?

If so, lets say we want to finish with a cube of 1" side, volume =1x1x1 cubic inches

Using the calculator from How and Why To Make A Shrinkage Measure https://www.ceramicartsqld.org.au/index.php/component/k2/how-and-why-to-make-a-shrinkage-measure

shrinkage.jpg.0e3c3897e2fda7953918fdb4be358d25.jpg

So the volume of the wet cube is 1.18x1.18x1.18 = 1.64 cubic inches

PS The calculator uses 1.18 = 1/(1-0.15) rather than 1.15 as the shrinkage is 15% of the wet size rather than the dry size.

 

Yes.  So 1.18 minus the 15% shrinkage is 1.003 inches.  Or effectively 1 inch.  

If you just add 15% to 1, that's adding 15% of the shrunk size to the wet.  That doesn't work.  

It's all correct.  

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6 hours ago, PeterH said:

PS The calculator uses 1.18 = 1/(1-0.15) rather than 1.15 as the shrinkage is 15% of the wet size rather than the dry size.

 

So there is a margin and markup argument here but I think I get your dilemma.  We definitely do not want to multiply by the cube of 115% or 1.15 ^3. I think that is pretty much gone from the argument. And if we look at it in other terms  let’s say You promise to give me 10% more fluid ounces of soda than your competitor and he was providing 100 oz. .

So you take 100 oz and we add 10 oz more, and I get 110 oz, or 100*1.10=110. Done deal, right?

But I come back and say When I went to school I learned that dividing by a fraction is the same as multiplying by the  reciprocal  so (1/2)/(2/3)= (1/2) * (3/2)  Or multiplying by 1.03 is the same as dividing by (1/1.03) So I Say you owe me 100 oz / .90 = 111.11 oz.

Well, I have been cheated! Not really, but it depend on perspective and the Series used to approximate this relative equivalency error is known for 1/(1-x) = 1+x+x^2+x^3 ..... or X^2/(1-x) 

I guess an argument can be made why they chose to divide by the reciprocal but this can add error. You would likely argue in our example above that you agreed to provide 10% more so multiply 100 by 10% and add it together. Of course if we deal in reciprocals my number 111.11 * .90 is 99.99999. So who is right?

As Liam said in the example above, multiplying by 1.18 x .85 is 1.003..... very close!

So my view, shrinkage likely will not be perfectly uniform and converting a fluid volume to a measure also a bit problematic as well. But for complex shapes or simple shapes it is probably  a decent approximation. Applying it this way is shapeless and universal. For potters, hopefully easy and straightforward and the important point that the interior volume needs to be this size or greater is being glossed over here. In my view something that should be relatively easy to get you very close to the size you will need.

So on the fly (Sitting at the wheel) , students say my clay shrinks 10% and I want my cup to be  At least 3” round (Inside / outside) when finished. A relatively simple way for them to do this is multiply 3” by 1.10 which is 3-1/3 inches. Bigger than a quarter, less than a half, close to 3/8” if they are using an imperial ruler. Most just throw to under 3-1/2”  and understand their clay does not shrink exactly by 10%. and their end result will be very close to 3”. Asking them to divide by the reciprocal would likely get me some weird looks and a lot of muttering under their breath, but gives very near the same answer.

Edited by Bill Kielb
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Not to throw a wrench into the mix but clay can shrink differently in different axis-say height vs width.So shape can matter a lot. The calculators are just a starting point really. Testing all the way thru until done with firing to an realatively exact temp is needed. Then with repeated tests would one really get it exact. There are so many variables -The shrinkage of clay depends on moisture content of that clay which can change batch to batch-The form matters ,wide, high,cloes in neck? the firing temp. Most of us potters consider its close enough. For those  of you who have this tiger by the tail I can send you any business I have turned away on making forms to exact volumes which over the past 40 plus years has been more than a few. Really slip work is the best method for reproduction of same volumes. Boring at best but very dependable. You still have to measure every aspect of the slip making to control the shrinkage .

In my mug discriptions they read like a meduim mug will hold 12-14 oz Same with a dinner plate they are about 10-10,1/4 inch diameter. If the bottle needs to hold exactly 12 oz  no more no less I'm out.Not worth the headache for me. Of course nowdays no custom work . I only make the forms I want to make ansd that seems to hover around 35 forms.

Edited by Mark C.
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On 10/2/2020 at 10:51 AM, Hulk said:

Might also consider functional vs. absolute capacity - where one fills to to in everyday use; I describe my vessel capacity thus.

Yep.  And, though we're usually talking about liquids, 'functional' capacity will generally be slightly higher for solids than for liquids..  For example:  A measuring cup with an 'absolute' capacity of 2 cups can usually be filled to the brim with flour or sugar, and carried to its destination with no spillage - but fill that same vessel with water, and you'll have a hard time moving it without sloshing some out... 

Or, a more common pottery application:  You can't put 12oz of coffee in a mug with an 'aboslute' capacity of 12oz unless you have super steady hands - or plan on slurping the first couple sips without picking it up.

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When customers ask for something of a specific dimension, I always tell them it will be within 1/4". I can usually get closer than that, but I won't guarantee great precision, as there are too many variables when it comes to shrinkage.

We used to run into precision issues at the glass shop I worked at after finishing grad school. People from the local university would call and ask for pieces of glass for whatever science experiments they were doing and would want it cut to within a millimeter or even less. Our policy was that we didn't work in metric measurements, and nothing closer than 1/16" inch. You've got to accept the limits of the material and your processes.

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