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Hey there.

Im a newbie here. Ive been working on creating some slip casting molds and I've had some mixed results. 

My troubles mostly have to do with mixing the plaster. Ive read Andrew Martin's Mold making and slip casting book but I refuse to use his plaster formula which uses metric, imperial, volume, AND weight. 

I've attempted to convert the formula several times now and I'm feeling pretty silly as I cant seem to get the results I'm looking for. 

I am using a NEW bag of #1 pottery plaster. 

Ive attached am image of a calculator I made in Sheets but my ratio is 2.25 Quarts H20: 6.75lb(Andrew Martin formula). I'm Canadian so I want this in metric and converted it. 2.13L(Water) 3.06Kg (plaster)

I sift my plaster into the water and let it slake for 3 minutes. I mix with a drill/paint mixer for 3 minutes. I tap to release air bubbles. I carefully pour into my mold. I've found that the consistency of the plaster is think, like lightly whipped whipping cream. It pours and sets into the piece I'm casting but mounds at the end. My plaster is hard after 12 minutes. 

After a week of drying I wet the mold and poured slip. I let it sit for 15 minutes then drained. I waited 2 hours and checked the casting, it was still too soft so I held off opening the mold. I came back 7 hours later to open the mold. The casting was 1/8" think and ripped in half when I opened. 

Based on Andrew Martins formula (calculate the cubic volume for water and x3 for the amount of plaster) and my results I'm thinking this formula is broken in some way. And Im at a loss as this happened 3 times so Im asking you smart and lovely people. 

If this has been covered or if I've posted in the wrong section please let me know as Im new here :) 


Thanks, mv


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I think there might be an oopsies with your conversion of plaster weights. 6.75 lbs of plaster would be 6 lb 12 oz, convert that over to metric and you get 2.77 kg of plaster.

This is going to give you a ratio of approx 0.66:1 water:plaster which is a slightly different than the USG target of 0.7:1  water:plaster

If your scales do imperial this chart is good, I like weighing the water so you don't have to convert the US quart used by Andrew Martin etc to the Canadian/imperial quart then to metric.

2 hours ago, DangerMDV said:

After a week of drying I wet the mold and poured slip.

This might not have been long enough. I know in my damp climate it takes much longer than that unless I use a dehumidifier.

Welcome to the forum.

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I've weighed a freshly made plaster mould, left it a week in the studio (greenhouse), weighed it again.  Very little difference.  Left it in a draughty place, in the sun, weighed a week later, quiet a bit lighter.

The mould needs to be really, really dry.

Also, different slips need different casting times and different setting/drying times. 

Time of year can make a difference of 3 times the casting time here.  

Don't give up, try to get the mould as dry as possible, don't wet the mould before pouring the slip


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Woah! Awesome, thank you everyone for replying.

So my error in converting the volumes from imperial to metric seems to be the issue. 

I'll adjust my calculator, remake my mold with the correct ratio 0.7:1 ratio with cold water, and wait until my mold is bone dry before pour slip. Its rainy season in Ontario so it might be a couple weeks before I can pour slip again. 


Thank you to everyone who chimed in. I really appreciate it. 





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Math is still off.

2.13 L = 2.13 kg (for all intents and purposes here) multiply that by 10 = 21.3 divided by 2.77 = 7.69  therefore plaster is still too high to meet the  (7:10) ratio.

7 kg of water : 10 kg of plaster (or 0.7kg : 1 kg)


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Ok. More oopsies.

I couldn't figure out the math (as presented above) so I reached out to a friend who is a high school math teacher. He helped explain it to me like this. 

  H20                 7               2.15(L)

--------    =    ------   =   -------- 

 Plater             10                 x(Kg)

Cross multiply

7x = 21.5

divide both sides by 7.

x = 21.5/7 

x= 3.08Kg of plaster. 

Third time's the charm? 


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42 minutes ago, DangerMDV said:

I couldn't figure out the math (as presented above) so I reached out to a friend who is a high school math teacher. He helped explain it to me like this. 

Just weigh out everything in grams. If your mold is 180 cubic inches: 180 x 16.316 = 2937 grams of plaster and 2056 grams of water (2937 x 0.7).

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Somewhere (on a different computer) I have a chart that has litres down the left column and kgs of plaster in the centre  column, and mould volume in the right column.  That way, I can choose a volume close to required, and read off weight of plaster and volume of water.  I don't weigh the water, the chart shows nice round numbers of litres.  

Easier to measure water in graduated jug than to weigh it.  Only have small scales at the studio.


Edited to add:  Found this in my gallery, have an updated one somewhere with nicer numbers in the water column.


Plaster : water ratios


Edited by Chilly
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As everyone has commented about your proportions I'll leave that topic alone. I will, however, comment on your process.

For 30 years I have found it best to let the plaster sit for 2 minutes. I then mix the plaster for 2 minutes. I then let the mixture sit for 1 minute.  If the mixture seems to be thickening nicely I will pour, if it still seems watery I will continue mixing for a minute or two.  I mix the plaster by hand using a jiffy mixer. (Not with a drill in other words) The companies, USG and GP, suggest using a drill to mix but I find that generates too many air bubbles. Rather than "tap the bucket" I find it best to swirl the bucket. (As you do so you will see bubbles come to the surface. ) If you're really annoyed by air bubbles you can lightly spray rubbing alcohol over the mix and that will break the bubbles.  If I'm mixing a large batch of plaster, 3 to 5 gallons, and holding the bucket is difficult, I will use a spatula to gently swirl the mix from within. (This will bring air bubbles to the surface.)

Water temp - slightly cold is best. (Not very cold.) Warm water speeds the set and very cold water slows the set. (I also found very cold water contributes to plaster warping.)

You haven't mentioned what kind of clay you're using? Porcelain doll makers typically wet the molds before casting. With stoneware casting slips I don't see the need. (In my experience stoneware slip takes longer to set up than porcelain.) Using very dry molds is best but with porcelain slip not always necessary.

You mention that the plaster is mounding toward the end of the pour. I would suggest you simply pour sooner. Plaster varies: old plaster sets up quick and needs to be poured quick, new plaster sets up slowly. Sometimes I increase my mix time, with new plaster, until I see the plaster thickening.

Once I've poured the plaster, in the mold box, I will then take a spatula and gently create a wave action in the setting mix. This will cause the plaster to level out and it will bring air bubbles to the surface. I then take a metal scraper and gently wipe the surface of the plaster. In most instances this will result in a very smooth mold surface.

Timing - Once I've added the plaster to the water I set the timer for 25 minutes. After 2 minutes I mix, after 2 minutes of mixing I let sit, after 1 minute of sitting I pour.  20 minutes later I can de mold. I then give myself about 15-20 minutes to prep for the next pour. (Assemble the mold parts, clean the mold box, weigh out plaster.) In this way I know each pour takes about 45 minutes.






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Thank you everyone who answered my questions and helped me find my way out. Im happy with my calculator and have used it a bunch to calculate materials needed and costs for a few upcoming projects. Not sure how I to share it with you (maybe I'll build a website? Would anyone be interested in using this?).

I don't have any potter friends, so finding a community of actively engaged people is VERY exciting and makes me want to learn more so I can contribute. Stay safe out there :)

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