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Old 07-16-2019, 08:17 AM
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Franklin Sankar Franklin Sankar is offline
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Estimating plaster mix

If I have 1 liter dry plaster or sand and add 1 liter of water , I donít get 2 liters. What is a rule of the thumb I can use to estimate the volume of the final mixture
Franklin
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Old 07-16-2019, 09:01 AM
Shawn Everette Shawn Everette is offline
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The water is going to absorb between the crevices of the plaster and sand particles, so you'll end up with less than the projected sum. I never had a great measure for estimating home brew, just kinda knew how much when I was doing a lot of casting. Always hedge on more, and have an extra bucket ready to go should you need to mix.
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Old 07-16-2019, 09:18 AM
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Thanks. I just tried a cup sand and a cup water and got 1 1/2 cups mixture.
Franklin
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Old 07-16-2019, 09:39 AM
Shawn Everette Shawn Everette is offline
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You'll generically end up better off doing measurements by weight rather than volume. Particulates can settle leaving you with variations each time if you measure by volume.

If you live in a area with a lot of humidity you'll have some weight variance, but it's usually minimal by comparison. When I was making a lot of casting I usually would only measure my dry ingredients and would add water till I "felt" the proper consistency because of the impact of material absorbing water from the air; same as when I was a baker.
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Old 07-16-2019, 10:32 AM
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At least You could taste it as a baker. 😀
Thanks
Franklin
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Old 07-16-2019, 10:57 AM
Steven O'Day Steven O'Day is offline
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Search Google for 'plaster calculator alfred'. WARNING: it is an xls file that will download automatically to your computer. The spreadsheet calculates the amount of plaster to water for a given volume.

The ratio is important, weigh both each time. You want consistent results. You might need to change the plaster/water ratio for your plaster/sand mix.

The comment on humidity is a good one, keep your plaster in a plastic bag or sealed container. Try to get fresh plaster, it will change over time and with air exposure.
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Old 07-16-2019, 11:58 AM
Shawn Everette Shawn Everette is offline
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Your shelf life for plaster is going to usually be 3-6 months, longer in dryer areas, less in moist. Climate control is your friend. Some refractories like castable are going to have the same type of time constrains, but inert ingredients like silica will last.

On a side note, a bowl full of old plaster that's been fluffed makes an excellent knock off for hot paperweights.
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Old 07-16-2019, 12:03 PM
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Henry to the rescue

The proportion of plaster/silica to water is critical in order to fabricate a strong mold. If you don’t add enough plaster/silica to the water, the ratio will be such that the mold will be soft and readily crack when subjected to heat. USG, a major manufacturer of plaster, recommends consistencies (proportions of plaster to water) and recommends that you weigh your materials. Author’s Note: The USG recommendations are for plaster to water and not for plaster/silica to water, but for our usage, we will use their calculations to estimate the ratio of plaster/silica to water. Weighing the plaster/silica to water defines the exact density of your mold.
Once you’ve calculated the volume of investment you need, it is necessary to convert this information in weights of plaster/silica to water. It is assumed that you have subtracted the volume of the model to be cast from the volume of the mold box into which you will be pouring your plaster/silica into. Remember, we’re working with cubic feet.
The next step is to select the consistency you will be using; low numbers equal higher density molds. The higher the number the faster the materials will set. It is really not necessary to go above 60 consistency. Most casters use 50 to 60 from the chart below. I use 55 and find this ratio quite adequate. Another important aspect of doing accurate measurement is that if you find it necessary to add more plaster/silica, the expansion and contraction of the different layers of your mold will be similar and you will not get any separation between those layers. For very large molds that require you to mix many batches of mold mix, this weighing method is very important.

Source. Free PDF on internet

Last edited by Franklin Sankar; 07-19-2019 at 07:13 PM. Reason: Add info on Source
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Old 07-16-2019, 12:05 PM
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Consistency
Plaster/Silica
Water
Total

50
78
39
117
Highest Density

55
73
40
113 lower density

60
68
41
109
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Old 07-16-2019, 12:44 PM
Shawn Everette Shawn Everette is offline
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As far as water is concerned it has a lot to do with the type of detail you are trying to achieve. A lot of times I would use a thinner skim coat to catch as much detail as possible, before using a thicker backing mix that had a handful of thin fiberglass for extra support. This was especially the case for something hand built vs using coddle boards.

Unless you are using some kind of embedded support or casting box I wouldn't recommend trying to invest in batches. There is only a small window of time before the plaster is too set to fully bond with a new mix. The hydraulic pressure will cause the layers to separate since the bonds are fairly weak, especially when heated. Scoring between layers can help, but physical adhesion to something like a chicken wire armature worked better.
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Old 07-16-2019, 12:59 PM
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Some nice hints. Thanks. I don't expect much success with my first attempt since it's mainly powders that was on the shelf since last year but I want to see how it reacts to the water and the refractory. My imagination goes wild.
Franklin
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Old 07-16-2019, 04:00 PM
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we do always seem to have one cat and we have bought "Delicat" as a food for said cats over the last two decades and we keep every container it comes in. MB keeps all her cane in them and a number of years ago, I started moving plaster and castables into them. They really keep the moisture out. I think at this point we have several hundred of them .Think screw lid containers in the studio.
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Old 07-16-2019, 04:23 PM
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I store mine in pig tail buckets. They have a rubber gasket in the lid.
I pit it in a plastic bag and then in the bucket. They donít last forever.
Franklin
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Old 07-20-2019, 05:21 PM
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Page97 ver4 glass notes recommends using a fine grain castable refractory to make a blow mold. Anyone tried it as yet?
My green cast 95 has some small stones. I was thinking about sifting it to make it fine grain.
Franklin
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Old 07-20-2019, 06:02 PM
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That is fine grained as is MORCO 95. Don't try to change it. Just add asmall amount of water.
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Old 07-21-2019, 06:45 AM
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Ok thanks
Franklin
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Old 07-21-2019, 04:28 PM
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I keep no.1 pottery plaster, and hydrocal wrapped in contractor bags (plastic garbage bags on steroids).

My approach is to use it as quickly as possible. If I have leftovers that have been open for more than a year (like I do now) I just throw it out, or possibly use it for something other than casting molds. It relatively cheap, so when I need to make some molds I go buy a bag or two at the pottery supply store. It's like $25 a bag.

I know not everyone has as easy access to materials. I now operate on a shoestring budget, so there are times when I have to work with what I have on hand, and just cross my fingers.

Point is if you have the resources buy it fresh.
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Old 07-23-2019, 03:09 PM
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What makes it stale. If itís moisture with no lumps then I am going to heat it up and drive out the moisture.
Franklin
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Old 07-23-2019, 03:23 PM
Shawn Everette Shawn Everette is offline
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Atmospheric moisture can trigger the activation of the plaster without much in the way of notice unless it's really bad. Once it's been done there not really anyway to undo it outside of getting a new bag. $25 for a new bag of plaster is a lot cheaper than loosing glass to a bad casting. Testing a small batch is usually a good protocol if anything is suspect.
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Old 07-24-2019, 09:28 AM
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your difficulty Franklin, in Trinidad, is being able to buy fresh plaster in the first place. Humidity is fast and furious and you are humid. If you can actually buy fresh plaster, which I doubt, transferring it into sealed containers is necessary.

I suspect you will be a bit better off using the refractory castables mixed with the silica. Getting a bag of calcium aluminate cement from Hi Temp in Fenton Missouri and getting it into sealed containers would really help. I use it to refresh aging castables. It comes in 25 lb bags and is really useful. About one lb in 25 lbs of castable will make it stiffen up. If does lower the performance temperatures of castables in high temperature envirionments like furnaces.
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Old 07-24-2019, 08:38 PM
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Thanks responses most appreciated.
Franklin
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Old 07-26-2019, 10:55 AM
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Quote:
Originally Posted by Franklin Sankar View Post
What makes it stale. If itís moisture with no lumps then I am going to heat it up and drive out the moisture.
Franklin
The thing to keep in mind with plaster is that it is a chemical reaction. This is why heat is generated during the setting process. Once the reaction starts it cannot be fully stopped or reversed.

Think of it like a two part epoxy. Mix parts A and B, and you've begun the process. You can slow it by temperature, but not stop it.
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Old 07-27-2019, 10:44 PM
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Well put.
Thanks
Franklin
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