Sizing Up Pottery, Or Making to Fluid Ounce Volume

[Polydeuces]

How do y'all go about this process? Do you go about it via trial & error? Or do you calculate before making a run of new forms/shapes?

I'm interested in making a couple prototypes to make molds from for slip-casting, and I'm shooting for a specific target volume. I found this calculator: https://photopottery.com/volume-calculator-cylinder.php which seems like it may be helpful, though I'm curious about others' experience and if anyone's got any tips for this kind of work.

Thanks!

[Gabby]

This calculator would work only for cylinders.

Maybe only because I am a math teacher, I'd go in knowing how much bigger I needed my dimensions to be to accomplish the larger volume.  Volume varies with the cube of the dimensions.  For example, if you wanted a container to hold eight times the volume of a container the same shape, you would increase each dimension by a factor of two. If you wanted to double the volume, you'd increase each dimension by a factor of cube root of two. 

[neilestrick]

It's a difficult thing to math out unless you're just making simple cylinders. Ultimately you just have to make a few of whatever form you're doing, keep good notes, and figure out how much clay you need and the finished dimensions for the size that works. I usually make a few, then fill them with water to see how much they hold.

[Polydeuces]

Thanks y'all. @neilestrick: When you say make a few, do you mean all the way to the finish (glaze fired)? Or do you test them while they're still green and take shrinkage into consideration? Doing my best to inform my process—thanks!

[Mark C.]

Make the pots all the way thru then measure-unless its a straight cylinder

This is wheel thrown work not slip-thats going to be a lot harder-but you could get close if you know your shrinkage rates.

[Polydeuces]

Alright then, thank you!

Final, million dollar question.

So if I'm making a form, I finish it and it goes all the way through—I've taken notes and have the dimensions pretty much locked in, and I have the correct fluid ounces.  When I go to make a mold of that form, is it best to throw a fresh one, and to mold from that?

It all seems like dreadfully tricky business! How exciting!

Thanks again!

[Mark C.]

Whatever you use to make a mold from it will shrink to whatever your slip shrinks-say 12%  for this example

So whatever your finished piece to take a mold from just keep in mind it would shrink 12% more when fired in finished state.

[Polydeuces]

Great! I think that's everything I need to know.

But there's one last thing I want to know: If you threw a form, and when it was leather hard quickly filled it and poured out the liquid into a separate container to be measured—would you be able to figure out the volume of the finished piece through the shrinkage? A part of me thinks it would work, as the volume would change at the same rate as the dimensions, right?

Thanks for all the advice!

[neilestrick]

If you know the amount it has shrunk at leather hard, and can do the calculations from volume to dimensions, then yes that would work.

[Chilly]

On 7/31/2018 at 12:16 AM, Polydeuces said:

Great! I think that's everything I need to know.

But there's one last thing I want to know: If you threw a form, and when it was leather hard quickly filled it and poured out the liquid into a separate container to be measured—would you be able to figure out the volume of the finished piece through the shrinkage? A part of me thinks it would work, as the volume would change at the same rate as the dimensions, right?

Thanks for all the advice!

At leather hard it would absorb some of that liquid, so you'd also have to weigh the form before and after filling/emptying to see how much had been absorbed.

When making a mould, the master needs to be as perfect as possible.  The finished casting will never be as good as the original, so spend lots of time getting the surface finish as good as you can.  Also, make your model much thicker than you would normally, or even solid, then it will hold up to the weight of plaster above and around it.

[fergusonjeff]

If you really need to determine the volume of leather-hard or even bisque-ware use something other than water (rice, beans, sugar, sand...)

[preeta]

What shape are you thinking of? A cylinder?

does a cup have to be exact? 

Ive watched videos of professional potters making and asked them questions to know (thanks IG) it differs from person to person. I make approximate. It’s more about the measure of clay (in throwing) than the height. A 12 oz cup could hold 13 or 14 oz but not less.  

Since you are slip casting I’d make a little more not common shape.  

Of course that is if you are making a simple 2 pc  mold. 

[Rae Reich]

On 7/30/2018 at 4:16 PM, Polydeuces said:

Great! I think that's everything I need to know.

But there's one last thing I want to know: If you threw a form, and when it was leather hard quickly filled it and poured out the liquid into a separate container to be measured—would you be able to figure out the volume of the finished piece through the shrinkage? A part of me thinks it would work, as the volume would change at the same rate as the dimensions, right?

Thanks for all the advice!

You could line the raw clay form with a thin baggie and pour in a measured amount - if it doesn't fit, you'll know before you overfill.  Then you wouldn't need to wet the form or weigh the baggie. 

Good luck!

[Rae Reich]

1 minute ago, Rae Reich said:

You could line the raw clay form with a thin baggie and pour in Your 12 or 16 oz - if it doesn't fit, you'll know before you overfill.  Then you wouldn't need to wet the form or weigh the baggie. 

Good luck!