Monday, August 2, 2010

Clay Wedging Table - plaster mixing ratios and guidlines

While I was at the Clay Studio last month, a few of the guys tackled pouring a new plaster top for the studio's large wedging table. It was quite a process to watch. I believe they used around 150 lbs of plaster!Mixing plaster has always been one of those things that makes me a little nervous and hesitant. Earlier this summer I poured plaster for my own new wedging table/kiln shelf storage cart. (Well, technically, my wonderful husband built the table/cart, and my good friend and artist Nigel Rudolph poured the plaster. See, told you plaster made me nervous.)
But today, I thought I would post some plaster mixing ratios and guidelines that I have found helpful....

The following guidelines are copied from a handout posted at the Clay Studio of Missoula:
(original source unknown. I also added a few notes, examples, and conversions)

Plaster Mixing Guidelines

Recommended Plaster: #1 Pottery Plaster

How do I calculate how much water or plaster I need?
Calculating volume for solid shapes
For rectangular shapes: Volume = Height x Width x Depth
For circular shapes: Volume = Pi x Radius2 x Height (Note: Pi is apx 3.14. and that's supposed to be radius squared, but I cannot get the text formatting to cooperate.)

Divide the volume by 80 to find the number of quarts of water you will need to make enough plaster.

How much plaster do I add to water for a proper mixture?
The recommended consistency is 7lbs water to 10lbs plaster. So a 7:10 ratio, by weight 2.85lb plaster to 1 qt (2lbs) water (Note: If you want stronger plaster, you can multiply the weight by 3. Softer plaster, multiply the weight by 2.75)

Example -
Your mold is 5" x 5" x 4" so your volume is 100 cubic inches.
100 divided by 80 =1.25. You need 1.25 quarts of water
Multiply 1.25 by 2.85 = 3.56. You need 3.56 lbs of plaster.

Conversions -
1/2 quart = 1 lb = 16 oz
1 quart = 2 lbs = 32 oz (above example 1.25 quarts of water= 40 oz. Multiply 1.25 by 32oz )

1 lb = 453.6 g

What method do I use to add plaster to the water?
Always add plaster to water not the reverse!

Sift plaster into water with a spoon or by hand. Continue doing so until a small mountain forms and remains above the water level. Once that mountain forms, let the plaster "soak" for 1-3 minutes. The greater the amount, the longer the soak - anything in a one gallon bucket needs only a minute or so.

Once the soak cycle is complete you can mix the plaster for a couple minutes. In order to get the most consistent mix, it is best to mix the plaster with a hand blender for small batches, or a drill with a mixing attachment for large batches. Mixing by hand is possible, but the smaller air bubbles tend not to release in the mix and might appear in the surface of your mold.

If mixed by a mixer, be sure to mix a bit after by hand to feel the consistency. The plaster is ready to pour in the mold once it changes from watery to creamy. Work quickly before the plaster starts to set.

Keri Radasch also has similar notes on her site and a link to Ian Anderson's guidelines as well.
Hope that helps. And if you're like me, the more times you do it, the easier (and less scary) it becomes. I'm actually looking forward to making a whole bunch of drape and slump molds later this month.

Happy Mixing!


-Rob, Simple Circle Studios said...

Good post; Thanks for the information. I too have always been a bit leery of messing with plaster. Maybe I will have to give it a go sometime.
And your wedging table/kiln furniture storage shelf is great! I am currently lacking a good wedging table and kiln shelf storage. Perhaps I can cobble together something similar.

Kings Creek Pottery said...

Now that is a NICE plaster table and cart he built for you! I think I'll point my husband in the direction of your blog.

Plaster always makes me a bit nervous too. All the caution of not getting plaster any where near the clay, etc. Like you, I try to avoid it as long as I can :)

Thanks for the recipes and info~


Anonymous said...

How timely. I was searching through my books late last night looking for this information. I'm checking my plaster quantity, and getting to it in the next few days. Thanks - naomi

cookingwithgas said...

Plaster- love it or hate it.
The first time we ever tried some it was a bad batch- scared us off for years.
Now we know one of the tricks is to make sure it is fresh and use right away.
Great post!

Ty said...


I've just started throwing and wasn't sure what the story is with plaster and clay? I've made my own plaster bat.... it's quite small but just use it for recycling clay. Is there a danger of clay and plaster?

Meagan Chaney said...

Hi Ty,

The concern with plaster and clay is that if large particles of plaster get mixed into your unfired clay, it could cause the fired clay to break. Here's a thread I found that explains it a bit better and with more detail...

Best of luck to you with your new throwing endeavour!