Slip Casting - Plaster Ratios

[DangerMDV]

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

 

[Min]

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.

[neilestrick]

7 parts water to 10 parts plaster, by weight. Weight is the easiest way to measure out both the plaster and the water, rather than converting the water to volume. You need 16.316 grams of dry plaster per cubic inch.

[Mark C.]

Your mix was off but its easy now with above post-7-10 by weight-Neil is on it.Alsocold water sets slower warm water faster-not sure water water temp you are using? I like cold water-powere mix for a few minutes drop bucket on floor to get air bubbles to top and pour .

[Min]

Yup, we are saying the same thing, 0.7:1 or 7:10 water:plaster.

[Chilly]

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

 

[DangerMDV]

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. 

mv

 

 

 

[Min]

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)

 

[DangerMDV]

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? 

 

[neilestrick]

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).

[Min]

1 hour ago, DangerMDV said:

Third time's the charm? 

Yup!

edit: not trying to beat it to death but it really is easier to weigh the water than measure in fractions of a Litre. Just use kg/grams in place of Litres, would be 2150 grams (or 2kg150gr)

[Chilly]

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.

 

 

[Callie Beller Diesel]

Using litres as a measurement instead of cubic inches is going to simplify the math as well. It’ll keep any conversions out of it.

[Chilly]

Another chart, with nice, round numbers for the water.  

[Jeff Longtin]

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.

 

 

 

 

 

[Denice]

I usually put a fan on my molds to help them dry,  the sun definitely helps.   I have put some in my oven towards the end to finish off the drying process.  Heating above 180 F degrees will damage the plaster,   it will reduce it's production life.   Denice

[Mark C.]

Fan is best as Denice says  fan and sun is better.

[Marcia Selsor]

Another note, while tapping the plaster you can use a spray bottle of diluted alcohol t break the surface tension of air bubbles as it is setting up. keep tapping until you don't have anymore bubbles surfacing.

Marcia

[DangerMDV]

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 

[Gigima]

On 5/2/2021 at 9:33 AM, Jeff Longtin said:

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.

 

 

 

 

 

 

[Gigima]

I know how to make molds but my interest in doll making has been driving me nuts. I cannot find where to buy materials to pour into the mold. I use hydrocal and ultracal for years along with, chavant, white and WED clay. should I just call Reynolds advanced materials and ask them? What if I want to make plastic dolls and porcelain. I have a brush able two part plastic and too thick to pour called shell shock.. the other plastic is too runny and may set up too fast. Thanks.

[neilestrick]

Most clay suppliers also carry pre-mixed casting slips.

[DangerMDV]

Gigima, Im not sure where you are or whats around you, but I’ve had success finding places near me (100km from Toronto). 
 

Ive found Pottery Supply House (Oakville). They have everything from clay, slip, glaze, tools, wheels, kilns, etc. 

Ive also worked with silicone and for that I needed to go to a sculpture specific supplier, Sculpture Supply Canada (Etobicoke). They have everything you who need to start molding plastics. Ive use a number of smooth-on products which are great. You’ll need to read the tech specs or get help from a dealer on the right materials for you as there are so many with different properties. This could mean silicone molds and Polyethylene dolls... This can get very pricey. But its rewarding and your molds, if done well, can last hundreds of runs. 

If you dont have anything nearby you can order most of this stuff online and have it delivered to you home now a-days. 

For resources on plastics I’ve seen great stuff from the Prop and Cosplay world. TONS of videos on youtube. Theyre generally excited to share their processes. 
 

I hope I was able to help. 

[Gigima]

Y'all are awesome. So I am in the USA... I am familiar with smooth on. So with porcelain I will need to fire it in a kiln?  And we have a place here that does pottery that I will call.

 

Thank you so so much!

[neilestrick]

3 hours ago, Gigima said:

Y'all are awesome. So I am in the USA... I am familiar with smooth on. So with porcelain I will need to fire it in a kiln?  And we have a place here that does pottery that I will call.

 

Thank you so so much!

Any type of clay will need to be fired in a kiln. You'll need to figure out what temp/cone the kiln owner is willing to fire to, and buy a slip that is made for that temp/cone.

If you haven't cast slip before, unlike casting plastic, silicone, plaster, etc which set via a chemical reaction, when casting clay the plaster mold needs to be clean and porous, as the mold has to absorb water from the slip in order for it to set.