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arpetrone

Mold Making & Casting Slip Mixing Questions

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2 minutes ago, Denice said:

How thick is the mold and is it flat on the back side,  did you use pottery plaster for the mold?   Denice

More details are needed to help as Denice says

How thick is the thinnest part

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I would say on a 12 inch long mold it could be thicker in the middle (overall). The other thing is when appling pressure  with a press mold it need to be on a flat supported surface. 

Plaster is pretty soft really. Caulk it up to learning.

The last point did you mix it acording to water and weight formulas as thats crital for strong plaster molds

Edited by Mark C.

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you only made that mold 5 days ago.  and yes, it must be supported so you do not do something that makes it react like a seesaw.  even a small item under it can do that if you press hard in only one place.

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IIt was on the floor when I was pressing.  I wonder if it just wasn’t fully cured even though it felt dry, it seemed to chip and flake quite easily unless that sounds just normal for plaster. And maybe I shouldn’t have pressed so hard on such opposite ends.

making the molds were so time consuming it’s very annoying it happened but oh well I guess.

i think I mixed it correctly, I went by the calculation of 16.something grams of plaster per cubic inch. I found that number in a thread here.  Then I multiplied the total grams by .7 for the water ratio.

Edited by arpetrone

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If the mold chiped and flaked something is wrong-old plaster? bad mix relationship( to much water) mold still wet.

get your mixing from USG as its the  direct source

here is the how to-pay attention to all the details in this PDF below

https://plaster.com/mixing-instructions/

here is the formula

7#s of plaster to 10#s of water

This PDF is for the formula only

http://www.usg.com/content/dam/USG_Marketing_Communications/united_states/product_promotional_materials/finished_assets/usg-no1-pottery-plaster-data-en-IG1366.pdf

Edited by Mark C.

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20 hours ago, arpetrone said:

Are you saying the clay will crack or the mold?

i waited until all the molds were dry (not cold) to the touch - today I was pressing some clay in one and it cracked right in half...Maybe I was pushing too hard? The mold is only about 12 inches long

I was saying the clay might crack.

 

If the mould has cracked, it's one of three things:

  1. Old plaster
  2. Mould too thin/not substantial enough
  3. Too much pressure/weight.

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Thanks, I did look at that second link beforehand and I basically did what the instructions did in the first link except I mixed by hand.

I hope you mixed up your #s of water vs plaster because I did the reverse as to what usg recommends.

I don’t know how to calculate from 7 pounds water and 10 pounds plaster to the exact amount I needed but unless the 16.314 (or whatever the exact number was) grams per square inch is way off then maybe the plaster I bought was old.

I’m thinking most likely the mold just too thin and I applied too much pressure

Edited by arpetrone

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No I did not mix it up

70 lbs. water/100 lbs. product (32 kg water/45 kg product)

I just dropped off  a zero off this formula above to make a smaller amout for you to use.

This relationship is what you use no mader how much you make up

If you did something else thats way the mold is weak. If the plaster is old that can also affect weakness.Use. NEW paster and use this formula-power mixing will help as well-use a paint stirer on a drill

 

Often in ceramics its  the details-live or die by them

Edited by Mark C.

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