Water / Powder Ratios
There is a lot to cover here so we've organized it for you by topic.
Alginate
There are two basic types of molds used with alginate: "The Bucket Mold" and the "Layup Mold".
In a Bucket Mold, the alginate is mixed and poured into some sort of rigid container. The body part (hand, etc.) is pushed down into the alginate and the bucket keeps the alginate from distorting.
In a Layup Mold, the alginate is mixed and put directly onto a surface like the face or torso. The alginate usually averages between 1/4 and 3/4 of an inch thick. This thickness of alginate is very rubbery and flops around like a deflated balloon, so it needs to have a structural shell wrapped around it to support the alginate. Most of the time this is made with several layers of plaster bandages. The larger the mold, the thicker this plaster bandage shell must be to support the increased weight of the alginate and the casting medium being used.
In general, we recommend a water to powder ratio of 4 to 1 (by weight)- four times as much water (by weight) as alginate powder (by weight). Please DO NOT use 4 cups of water to 1 cup of powder. Water is MUCH heavier than alginate powder so this ratio is more like 11 to 1 by weight. Your alginate will come out VERY thin and just about useless. It will also take FOREVER to set.
For certain types of jobs, it is a good idea to modify the standard water/powder ratio. Our 590, 680, 880, and GENESIS formulas already have our low-flow properties, but this can be easily modified to suit your project. For example, we recommend a 3.75 to 1 ratio when doing a face, head or torso mold. This makes the alginate even less "Drippy", making the procedure much easier and less wasteful.
It is possible to "stretch" our Bucket Mold alginates (380, 390, 570 and 580) by mixing them a little thinner than "recommended". Please do a test first to determine how much thinner is going to work for you. As you add more and more water, the alginate will do three things. 1) It will take longer to set, 2) It will "set" softer, and 3) It will shrink faster after it sets.
We have heard of customers "stretching" their alginate to 5:1 or even 6:1.
You might be interested to know that our competitors recommend 2:1 or 3:1 ratios so their alginates don't "go" as far.
Measuring Alginate-
We recommend you use a scale to weigh out your alginate and water. Even an inexpensive postal scale is usually accurate enough to do the job.
Some people like to measure their alginate by volume. This is easier but not as accurate because alginate is a "compressible powder". This means it can be "fluffed" or "compressed" or anywhere in between so the weight of "A Cup" can vary by as much as 30%.
| Accu-Cast 590-IBG | Fluffed | Compressed |
| Weight of 1 cup | 2.6 ounces | 3.4 ounces |
If we use an "average" density of 3 ounces per cup, then we should mix this with 1.5 cups (12 fl.oz.) of water to achieve our desired 4:1 (by weight) water/powder ratio.
Water is very easy to measure by volume because it is NOT compressible. There is an old saying,
How much alginate does it take to do a hand cast? (We hear this all the time.)
The answer is (drum roll, please....): "IT DEPENDS". It depends on how big your "Bucket" is.
The first step is to decide on your bucket. The bucket needs to be big enough so the hand(s) fit inside (in the pose you've selected) without touching either the sides or the bottom. The alginate fills the space BETWEEN the hands and the inside of the bucket so you want to get one that is "as small as possible" while still having sufficient room.
The second step is to fill your bucket with water (do this in the sink and it will be much neater). Put your hand(s) into the bucket and let the water overflow into the sink.
The third step is to measure the REMAINING water. As we said above, each pint (16 fl.oz.) of water weighs 1 pound.
| Water amount | Water weight | Alginate weight |
| 1 pint | 1 pound | 1/4 pound (4 ounces) |
| 1 quart | 2 pounds | 1/2 pound (8 ounces) |
| 1 gallon | 8 pounds | 2 pounds |
Use the table above, plus your math skills, to calculate how much alginate you'll need.
If you're doing a Layup Mold, there is a way to calculate how much alginate you'll need for your job.
Our alginates, mixed at a 4:1 ratio take up about 137 cubic inches per five pounds of alginate mixture (that's one pound alginate mixed with 2 quarts (4 pounds) of water).
A typical layup mold will average approximately 0.25 inches thick, so five pounds of alginate mixture will cover approximately 548 square inches. This assumes NO waste, so if we estimate 10% waste, this figure drops to more like 490 square inches.
A typical face is approximately a circle with a diameter of 12 inches- or 113 square inches. Accordingly we see that a face mold will take a little less than 4 ounces (1/4 pound) of alginate (mixed with 1 pound of water).
You should do your own measurements, but here are some approximations for you to use:
A torso is between 400 and 600 square inches for shoulders to crotch. This would be between .81 pounds (13 ounces) to 1.22 pounds (20 ounces). Each leg will be between 200 and 300 square inches, for a total of between 400 and 600 square inches.
With these estimates, a small model would have approximately 800 sq.in. of skin on the front from neck to ankles. The back should be about the same. A large model would have up to 1200 sq.in.
Small model= ~800/490= 1.6 pounds (26 ounces) of alginate
Large model= ~1200/490= 2.44 pounds (39 ounces) of alginate
For both front and back, double these figures.
As handy as these numbers are, REMEMBER that it is always better to have a little EXTRA alginate mixture than to have "Not Quite Enough......"
LiquiStone
There are two basic types of molds used with Alginate and Gypsum Cement, the "Bucket Mold" and the "Layup Mold".
Filling the bucket mold requires filling the entire cavity left behind by the hand (or other body part). The LiquiStone needs to be fairly thin for pouring this type of mold because you want it to flow freely down to the fingertips and trap as few air bubbles as possible.
Filling a Layup Mold requires that you cover the surface of the alginate to capture the anatomical details. In most instances, a thinner "brush coat" is applied to the alginate itself and worked down into the detail areas to minimize air bubbles and voids. Then a thicker layer is added to add strength to the casting. This more viscous second coat should be applied to the first "brush coat" about 1 hour later. Unlike alginate, LiquiStone will stick to itself.
Build up the edges to make them less likely to break if they are banged. You can also add pieces of burlap to the second coat of LiquiStone to stengthen and lighten the casting.
As we said above, a thinnish mix is used for filling a Bucket Mold and for the brush coat of a Layup Mold. A thicker mix is used to build up the thickness of a Layup Mold. NOTE: With LiquiStone, little changes in mixing ratio make big differences in viscosity.
| Water/Powder Ratio (by weight) | ||
| Thinnish Mix | 1 to 2.3 (yes, more powder than water) | Example: 1 cup water to 18.4 ounces of LiquiStone |
| Thicker Mix | 1 to 2.6 | Example: 1 cup water to 20.8 ounces of LiquiStone |
LiquiStone is slightly compressible but not nearly as much as Alginate. It IS acceptable to measure out LiquiStone volumetrically. Each level CUP (8 ounce measurer) weighs about 9.5 ounces.
| Amount of LiquiStone | water @ 1:2.3 ratio (thinnish) |
water @ 1:2.6 ratio (thicker) |
| 1 cup- 9.5 ounces | 4.25 fl.oz. | 3.75 fl.oz. |
| 2 cups- 19 ounces | 8.5 fl.oz. | 7.5 fl.oz. |
| 3 cups- 28.25 ounces | 12.75 fl.oz. | 11.25 fl.oz. |
| 5 cups- 47.5 ounces (~3 pounds) | 21.25 fl.oz. | 18.75 fl.oz. |
| 10 cups- 95 ounces (~6 pounds) | 43.5 fl.oz. | 37.5 fl.oz. |
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