Nice work. Your 70% water to plaster ratio is pretty close to the 67% (2:3 ratio) recommended by Andrew Martin in his excellent “Mold making and slip casting “. As far as mixing time is concerned, you will have more working time if you clean your mixing paddle. Gypsum (cured plaster) catalyzes the set. See Weiser and Moreland J. Physical Chemistry 1932, 36, 1, 1-30. They also point out the greater initial strength of plaster mixed longer, but say after 10 days of cure the plaster mixed as short as 10 seconds has become equally strong.
Thanks @Alan S. Yes his book is a great resource and I found it after my tests. You're correct on pg27 he talks about rounding off the manufacturer's ratio from 70:100 to 2:3. Given that I'm weighing everything keeping with the original ratio is just as easy, but I think it does show there's some leeway. Thanks for the citation - it's always fun to read old papers. Looking quickly, I only see one reference to catalyst. However the paper does talk about different additives effecting the plaster. The only reference I see to 10s stirring refers to time to set, fig 4 (I don't see associated strength measurements). In contrast, in "PLASTER MIXING PROCEDURES USG PLASTERS AND USG HYDROCAL® BRAND GYPSUM CEMENTS - IG503", Fig 3 demonstrates the relationship between mixing time and compressive strength. I'm not sure the difference is actually all that relevant for use in pottery but does seem to exist.
Fantastic test, thank you. You have saved me a lot of time in doing my own experiments. I wanted to cast some plaster tomorrow of a new mug I have made a wooden blank for and this has helped me. 👍
Glad I could help! Luckily, these numbers also line up with some of the recommendations I got on comments in my last video, but having some direct evidence myself was very helpful.
Woot! Great timing. My sweetie, who is a potter, and I are planning to do some plaster pouring today! Thanks for the hard work you put into testing, I will use that info!
this was SO helpful!!! How did you not get any bubbles in the plaster as you were mixing it with the drill? I find that creates so many bubbles for me...
I'm actually still fighting bubbles myself! One of the best options is to have the right size container. A bit counter-intuitively, it is easier to mix up larger volumes of plaster. A lot of people suggest mixing by hand but that results in much weaker plaster and isn't what the manufacturer recommends.
Another thing you can try is adding a small amount normal school glue to your water and let it dilute (The water should be semi-cloudy but not thickened in any way) before mixing with the gypsum and pouring. It's an old potters trick for making weak/cheaper plaster stronger for long-term mold use. I can't see why it wouldn't improve stronger plasters too.
That's an interesting tip I hadn't heard of before. I would imagine it would make it stronger. I guess it also depends on your final use of the plaster as well. From what I understand, there some magic structural properties that wick away water, and that is one of the things that changes with different ratios, mixing times, and even over time.
I would wonder how the glue affects the porosity and absorbtiveness of the plaster. I have been wondering why commercial molds are so much more hard and durable than mine. My plaster has been soft, chalky, and easily damaged, so I am looking for answers. I have thought about possibly adding some white Portland cement to increase the strength.
Hello! Can I use plaster as slip cast. I can't find any info about it. Most is making the plaster molds. But nothing about using the plaster itself in a liquid state as slip cast. Casting with plaster. Maybe playing with the ratio of water and plaster powder 🤔 . I want to make plaster figurines using silicone molds and I want the figurines hollowed. Thank you in advance!
No plaster does not work like slip. I would look at rotocasting, I'm guessing that might work with plaster? But not anything I have any experience with.
Thanks for catching that! I'm not sure what happened there. I've added them. And just in case they're here: www.usg.com/content/dam/USG_Marketing_Communications/united_states/product_promotional_materials/finished_assets/plasters-gypsum-cements-for-ceramics-application-guide-en-IG526.pdf www.usg.com/content/dam/USG_Marketing_Communications/united_states/product_promotional_materials/finished_assets/plaster-mixing-procedures-application-en-IG503.pdf
I have considered trying adding white Portland cement to my plaster to increase strength. I'm not sure how that would affect the absorption quality of the mold.
Nice work. Your 70% water to plaster ratio is pretty close to the 67% (2:3 ratio) recommended by Andrew Martin in his excellent “Mold making and slip casting “.
As far as mixing time is concerned, you will have more working time if you clean your mixing paddle. Gypsum (cured plaster) catalyzes the set. See Weiser and Moreland J. Physical Chemistry 1932, 36, 1, 1-30.
They also point out the greater initial strength of plaster mixed longer, but say after 10 days of cure the plaster mixed as short as 10 seconds has become equally strong.
Thanks @Alan S. Yes his book is a great resource and I found it after my tests. You're correct on pg27 he talks about rounding off the manufacturer's ratio from 70:100 to 2:3. Given that I'm weighing everything keeping with the original ratio is just as easy, but I think it does show there's some leeway.
Thanks for the citation - it's always fun to read old papers. Looking quickly, I only see one reference to catalyst. However the paper does talk about different additives effecting the plaster.
The only reference I see to 10s stirring refers to time to set, fig 4 (I don't see associated strength measurements). In contrast, in "PLASTER MIXING PROCEDURES USG PLASTERS AND USG HYDROCAL® BRAND GYPSUM CEMENTS - IG503", Fig 3 demonstrates the relationship between mixing time and compressive strength. I'm not sure the difference is actually all that relevant for use in pottery but does seem to exist.
Good approach to isolating and testing the variables. Nice video, very informative!
Thank you!
Fantastic test, thank you. You have saved me a lot of time in doing my own experiments. I wanted to cast some plaster tomorrow of a new mug I have made a wooden blank for and this has helped me. 👍
Glad I could help! Luckily, these numbers also line up with some of the recommendations I got on comments in my last video, but having some direct evidence myself was very helpful.
..thanks for showing this clearly and systematically..very helpful!!
Glad you enjoyed it!
Woot! Great timing. My sweetie, who is a potter, and I are planning to do some plaster pouring today! Thanks for the hard work you put into testing, I will use that info!
You are so welcome!
this was SO helpful!!! How did you not get any bubbles in the plaster as you were mixing it with the drill? I find that creates so many bubbles for me...
I'm actually still fighting bubbles myself! One of the best options is to have the right size container. A bit counter-intuitively, it is easier to mix up larger volumes of plaster. A lot of people suggest mixing by hand but that results in much weaker plaster and isn't what the manufacturer recommends.
Another thing you can try is adding a small amount normal school glue to your water and let it dilute (The water should be semi-cloudy but not thickened in any way) before mixing with the gypsum and pouring. It's an old potters trick for making weak/cheaper plaster stronger for long-term mold use. I can't see why it wouldn't improve stronger plasters too.
That's an interesting tip I hadn't heard of before. I would imagine it would make it stronger. I guess it also depends on your final use of the plaster as well. From what I understand, there some magic structural properties that wick away water, and that is one of the things that changes with different ratios, mixing times, and even over time.
I would wonder how the glue affects the porosity and absorbtiveness of the plaster. I have been wondering why commercial molds are so much more hard and durable than mine. My plaster has been soft, chalky, and easily damaged, so I am looking for answers. I have thought about possibly adding some white Portland cement to increase the strength.
Wow, thank you for doing all this work
k
Sure thing!
THis was great for me to watch thank for you time !!!!!
Glad you enjoyed it!
Hello! Can I use plaster as slip cast. I can't find any info about it. Most is making the plaster molds. But nothing about using the plaster itself in a liquid state as slip cast. Casting with plaster. Maybe playing with the ratio of water and plaster powder 🤔 . I want to make plaster figurines using silicone molds and I want the figurines hollowed.
Thank you in advance!
No plaster does not work like slip. I would look at rotocasting, I'm guessing that might work with plaster? But not anything I have any experience with.
Kent, in the description the link to the data sheets is missing.
Thanks for catching that! I'm not sure what happened there. I've added them. And just in case they're here:
www.usg.com/content/dam/USG_Marketing_Communications/united_states/product_promotional_materials/finished_assets/plasters-gypsum-cements-for-ceramics-application-guide-en-IG526.pdf
www.usg.com/content/dam/USG_Marketing_Communications/united_states/product_promotional_materials/finished_assets/plaster-mixing-procedures-application-en-IG503.pdf
Whow is the result if we mix gypsum, cement and water?
This is just (pottery) plaster and water.
I have considered trying adding white Portland cement to my plaster to increase strength. I'm not sure how that would affect the absorption quality of the mold.