Sorry, volume weight problem again...

[Katia T.]

Hi
I am trying to make a plaster/silica mold.
I have searched the archives - new and old, and I am still confused...
If I measure 1 litre of water I have to mix 1 litre of plaster and 1 litre of silica? I will have a 3 litres mixtur?
Am I right?
Thanks
Katia

[charlie holden]

No it won't work out to 3 litres since the powder has lots of air mixed in between the grains. You can't look at a litre of powder and tell how tightly it is packed. Because of this we usually measure powder by weight.

Most people mix 50% plaster and 50% silica by weight. I think it is good to mix them dry first. Mix them very well and store them in an air tight container.

I then mix 1.75 parts plaster/silica to 1 part water by weight. It is easy to measure the water since 1 litre weighs 1 kilogram.

Some people do mix by volume only. But I think they have lots of molds break if they make big molds that hold lots of glass.

ch

[Katia T.]

I then mix 1.75 parts plaster/silica to 1 part water by weight. It is easy to measure the water since 1 litre weighs 1 kilogram.
ch

Hi Charlie
You are the first person that do not speak that it is 1 part of water to 2 parts of the mix. I tried todat this one part and the result was a mess that I could not pour in anything...
I will try 1.75 tomorow
Katia

[Katia T.]

Hi
I´m trying to make a plaster/silica mold, but it seems impossible
I´ve made the mix of plaster and silica by weight. Mixed very well...
Then I tried to add it to 1 liter of water - result was a heavy and very very thick mess...
I tried 1,5 litres and 1,75 litres the result of both was a mess that would never dry... As I read at Fenton´s book, I did something completelly wrong.
I don´t now what else iI can try.
The worst of all is that one year ago I did lots of this molds for vase slumping, and never had any problems.
Please help me!!!
Katia

[charlie holden]

Hi
I´m trying to make a plaster/silica mold, but it seems impossible :?:
I´ve made the mix of plaster and silica by weight. Mixed very well...
Then I tried to add it to 1 liter of water - result was a heavy and very very thick mess...
I tried 1,5 litres and 1,75 litres the result of both was a mess that would never dry... As I read at Fenton´s book, I did something completelly wrong.
I don´t now what else iI can try.
The worst of all is that one year ago I did lots of this molds for vase slumping, and never had any problems.
Please help me!!!
Katia

How much plaster/silica dry mix did you try to mix with 1 litre of water? It should be 1.75 kilograms dry mix. If you want a softer mold go down to 1.7 or 1.6 parts dry to water by weight.

If you are making slumping molds rather than casting molds, they might not need to be so strong. If that is the case you can probably just use a given amount of water without measuring the plaster/silica. To do this you sift the plaster/silica into the water until it peaks -- little dry hills sit on top of the water without sinking in quickly. Let the mix slake -- let it sit there for five minutes while the plaster soaks up water. Then you can begin stirring it.

There's a set of charts that Bullseye puts out that has the correct amounts of plaster/silica and water for a range of mold sizes. Go to this page:

http://www.bullseyeconnection.com/getsmart/index.html

and download "Tipsheet 5: Box Casting". The last page is the one with the charts.

ch

[Delores Taylor]

Hi
I´m trying to make a plaster/silica mold, but it seems impossible
I´ve made the mix of plaster and silica by weight. Mixed very well...
Then I tried to add it to 1 liter of water - result was a heavy and very very thick mess...
I tried 1,5 litres and 1,75 litres the result of both was a mess that would never dry... As I read at Fenton´s book, I did something completelly wrong.
I don´t now what else iI can try.
The worst of all is that one year ago I did lots of this molds for vase slumping, and never had any problems.
Please help me!!!
Katia

Charlie's given you some excellent advise but perhaps there is other issues at play. Are you using #1 pottery plaster to 200 mesh silica and weighing each ingredient? You must first do your 50/50 mix by weight then add the water into your bucket and sprinkle to a dry riverbed consistency which if you weighed the 50/50 mix would come out verrrry close to the ratio Charlie discussed in prior posts. Mold will sometimes fail if the plaster is old or you get too much silica in your mix and it get gummy and won't set up right. Being more explicit about your mix being a mess might help us figure out why. Best wishes in your castings it can be very frustrating but it's really worth all the efforts.

[Katia T.]

[quote="Delores
Being more explicit about your mix being a mess might help us figure out why. Best wishes in your castings it can be very frustrating but it's really worth all the efforts.[/quote]

Hi Delores
What is happening is that the mix seems a thick mud , and it never dries..
Today, as I read at Fenton´s book, I tried a mix of 1 part water, 1 part plaster and 1 part sand. The result seems to be fine. It was easy to mix , and dried as expected ( I could remove the mold without problem). I don´t know if it will stand high temps for casting...
What am I doing wrong with the silica????
Thanks
Katia

[Delores Taylor]

[quote="Delores
Being more explicit about your mix being a mess might help us figure out why. Best wishes in your castings it can be very frustrating but it's really worth all the efforts.

Hi Delores
What is happening is that the mix seems a thick mud , and it never dries..
Today, as I read at Fenton´s book, I tried a mix of 1 part water, 1 part plaster and 1 part sand. The result seems to be fine. It was easy to mix , and dried as expected ( I could remove the mold without problem). I don´t know if it will stand high temps for casting...
What am I doing wrong with the silica????
Thanks
Katia[/quote]

I can think of several things that I've found problematic in the past. The first is your measuring by volume not by weight. As Charlie mentioned you'll get too much silica in your mix by using the volume method. You should weigh each amount of plaster and silica in a 1:1 ratio. The fact you mentioned it doesn't dry tells me it's the silica that's the problem. I live in a humid climate and when it's the rainy season (all but two weeks in August) and I'm at the end of my 50/50 mix sometimes the silica has settled to the bottom and I get a gummy mold. I think this is from not continually stiring my container I store my mix in (Charlie also advised to make sure you dry mix very well which is important).

Is your climate very humid? Humidity will come into play but it'd have to be fairly high for that to inhibit your molds setting up. That being said I've had that as a problem because I'm in a very humid climate but it generally is a much safer bet (90% that it's just too much silica and if you use a scale and weigh you won't have any more problems). Try by weight and see if that doesn't help and be sure you dry mix thoroughly. You should get the mix to dry hard not be mud it's only when you slake you get a dry riverbed look, then you mix your ps and water and it sets up and becomes hard. This presumes you don't have old plaster/silica which may be moisture damaged. Also I store my plaster/silica in plastic moisture proof tubes. As I write it's chucking it down with rain I'd never get a mold to sit if I left my plaster/silica in a bag and exposed to my humidity. Also if you have your paper bags on a concrete floor that wicks moisture and then transfers that to the paper bags, then the contents, this is a problem. When you buy new supplies always protect it in a plastic garbage bag or in a moisture proof bin which is how I work. I buy 50 pounds of plaster and silica and have a big tub. If you go with that volume continually give the entire tub a stir as the ultra fine silica does settle and needs to be dry mixed on a regular basis. One other last thought check to see if there are dates on your bags it might be too old when you first bought your supplies. The very best control is when you weigh your plaster, silica and water. I cheat and don't measure my water as I can visually tell but since you're having such problems I'd go to the web site of the plaster manufacturer and see what their exact water to plaster ratio is (it varies among the alpha and beta plasters) and follow their instructions. Maybe your plaster requires less water to set.

PS Experiment with the manufacturers ratio of water to plaster based on the exact product you purchased. My manufacturer even had a plaster calulator a paper slide rule gizmo that told you the various amounts of plaster to water based on the type of plaster purchased which I think is on the US Gypsum website but might not be helpful depending on the manufactuer's brand you are using.

Hope that helps...

[Katia T.]

Hi Delores
I received today a new package of plaster and silica. You may disbelieve, but I can´t buy silica here in Rio de janeiro, I have to buy it in São Paulo -thank God for the net...
Tomorow I´ll throw off all the old stuff and try all over again.
What do you think will happens with the plaster / sand mold I made today? I think I´ll try to put it in the kiln when it´s cured...
Thanks for your help and patience.
Katia

[Delores Taylor]

Hi Delores
I received today a new package of plaster and silica. You may disbelieve, but I can´t buy silica here in Rio de janeiro, I have to buy it in São Paulo -thank God for the net...
Tomorow I´ll throw off all the old stuff and try all over again.
What do you think will happens with the plaster / sand mold I made today? I think I´ll try to put it in the kiln when it´s cured...
Thanks for your help and patience.
Katia

Dicey but there's urban legend that pure plaster molds have survived. If it's lots of money to cast in glass I'd not take that chance. If it's not a huge cost issue it'd be worth the experiment.

[Katia T.]

Hi
My new silica arrived today, and I made some beautiful molds!
The problem was bad plaster or bad silica...
Thanks a lot.
By the way, the mold I made using plaster / sand / talc worked fine, no cracks at 870 celcius.
Katia

[Bert Weiss]

Delores

I was under the impression that the ideal mix has 40% plaster by weight and that doing it by volume, more or less approximates that figure. I think the idea is to have just enough plaster to get it to set up hard enough to work.

When I make up recipes by weight, I leave the plaster at 40% and mess around with the various additives.

[charlie holden]

The "Casting With the Best" cd is revealing in this regard. They experimented successfully with some mixes with less than 30% plaster by weight. One of the basic formulas was plaster/olivine/ludo -- 30/30/30 with maybe 5% kaolin. I haven't had the time to really study it and take it apart yet. The cd is kind of hard to navigate. I wish I could afford to print it.

One thing I have a little problem with is that they tested the mixes by casting cones of glass into cylindrical molds. This means that the mold is very thick at its bottom where the glass is very thin. I'm not sure it gives an accurate model of how strong the mold would be if all the mold walls were the same thickness. You still have to test your own mixes.

ch

[Katia T.]

The "Casting With the Best" cd is revealing in this regard.
ch

I was not able to buy this CD.
HWere did you get it?
Thanks
Katia

[Delores Taylor]

Delores

I was under the impression that the ideal mix has 40% plaster by weight and that doing it by volume, more or less approximates that figure. I think the idea is to have just enough plaster to get it to set up hard enough to work.

When I make up recipes by weight, I leave the plaster at 40% and mess around with the various additives.

Many people haave many different receipes based on their different casting methods. When I first learned this method I did 1 part plaster to 1 part silica by volume which turns out to be more silica by weight I would get cracks in my molds which in turn scarred my piece. I then went to by weight and eliminated my cracks. I've used this method for hundreds of molds and have had not problems. It's also a bullet proof formula for new to advanced students in my classes. The caveat is that once you get too much glass mass you have to beef up the mold and I use vermiculite once I exceed 100 grams for my model wieght.

Charlie--- ludo is crushed up old fired plaster so that would be a 70% plaster to 30% silica ratio with might be too tough a mix for delicate pate de verre but it might be the holy grail for large cast pieces that weigh 40 plus pounds. I'm not saying either of you are wrong but the question was in context of pate de verre and I know this formula works on a consistent basis. Longer holds and higher temps of cast pieces require different mold mixes.

Frederick Shuler's studies on mold materials reviewed 100%, 80/20,60/40 and 50/50 mixes and the shrinkage of plaster-silica factor was the smallest in the 50/50 range. Also he analysed the strengths of plaster and hydrocal. Hydrocal being .05% stronger which is not a sufficient enough amount for me to go with the extra expense and deal with the extra outgasing problems from hydrocal. If you're into doing more venting to compensate for the outgassing then hydrocal slightly improves the mix.

Just my opinion but I'm sticking to it. That being said once I do mega pieces that also include solid mass I'm sure I'll need another mix but for moderate size work of 16" and under, hollow core vessels... this breaks away clean, is not so hard as to damage delicate parts and it's worked for myself and students who love pushing the envelop with their wierd experimentations that have been a shear pleasure to observe.

[Katia T.]

Delores

I was under the impression that the ideal mix has 40% plaster by weight and that doing it by volume, more or less approximates that figure. I think the idea is to have just enough plaster to get it to set up hard enough to work.

When I make up recipes by weight, I leave the plaster at 40% and mess around with the various additives.

Wich additives? To strength the mold?
Katia

[Bert Weiss]

Delores

I was under the impression that the ideal mix has 40% plaster by weight and that doing it by volume, more or less approximates that figure. I think the idea is to have just enough plaster to get it to set up hard enough to work.

When I make up recipes by weight, I leave the plaster at 40% and mess around with the various additives.

Wich additives? To strength the mold?
Katia

The idea of strengthening the mold is a complicated one. Plaster is the binder that holds the mold together. The larger particles like grog or sand serve to strengthen the mold in 2 ways. They provide pathways for moisture to leave without cracking the mold, and they make a matrix of different particle sizes which is stronger than one of more similar sizes.

The confusing factor is that if you use too much plaster it can weaken the mold by promoting cracking.

I am surprised by Delores's experience that more plaster can stop cracking. That goes contrary to my understanding, but I won't argue with experience.

Here is a recipe I used to make molds that didn't crack. There is one catch here which is that I used an unavailable material, that I call perlite ore. It is actually unpopped perlite ore. Perlite is made by heating an ore up to 1500ºF when it pops like popcorn. The stuff I used is the ore that doesn't pop in to perlite and it falls to the floor beneath the machine. I discovered it while visiting a perlite factory looking for acceptable particles to mix in with my plaster. At any rate I would substitute olivine or grog. Grog sticks to the glass, so it has to be protected with a splash coat without it. I don't know how olivine behaves.

Water/ Plaster= 1/1.5 by weight

Plaster Mix10 lb
4 lb 40% Plaster
2 lb 20% Silica
2 lb 20% Perlite ore
1 lb 10% Diatomaceous Earth
1 lb 10% EPK

[jerry flanary]

Hey folks

Bert, About the comment from Delores, "Charlie--- ludo is crushed up old fired plaster so that would be a 70% plaster to 30% silica ratio with might be too tough a mix for delicate pate de verre but it might be the holy grail for large cast pieces that weigh 40 plus pounds. I'm not saying either of you are wrong but the question was in context of pate de verre and I know this formula works on a consistent basis. Longer holds and higher temps of cast pieces require different mold mixes" According to Thwaites the ludo may cut down on cracking because a) the ludo has various sized particles and helps with packing and b) chunks of ludo act as aggregate to stop cracks running through the mold. Also ludo is classified as a refractory or a modifier rather than a plaster (or binder) because the gypsum has already "gone off" and has no binder abilities (though it does speed up set time.)

Katia, can't you just get your own silica off the beach in rio? Or is it too hard to sift out all the cigarettes and thong bikinis?

[Katia T.]

Katia, can't you just get your own silica off the beach in rio? Or is it too hard to sift out all the cigarettes and thong bikinis?

Well, now I am completely lost

I am used to buy silica(flint, quartz) as a powder so thin as the plaster. On the beach we have sand.
I know sand is silica, but I always thought that silica was a very well "worked" sand.
Please, enlighten me!!!!
Thanks
Katia

[Katia T.]

Hey folks

Bert, About the comment from Delores, "Charlie--- ludo is crushed up old fired plaster so that would be a 70% plaster to 30% silica ratio with might be too tough a mix for delicate pate de verre but it might be the holy grail for large cast pieces that weigh 40 plus pounds. I'm not saying either of you are wrong but the question was in context of pate de verre and I know this formula works on a consistent basis. Longer holds and higher temps of cast pieces require different mold mixes" According to Thwaites the ludo may cut down on cracking because a) the ludo has various sized particles and helps with packing and b) chunks of ludo act as aggregate to stop cracks running through the mold. Also ludo is classified as a refractory or a modifier rather than a plaster (or binder) because the gypsum has already "gone off" and has no binder abilities (though it does speed up set time.)

Who is Thwaites?Thanks
Katia