Alginate Techniques

How Much Alginate Do I Need?

Baby Hand Cast ~3 ounces
Child Hand Cast ~6 ounces
Adult Single Hand ~1 pound
Adult Double Hand ~1.3 pounds
Face Cast ~4 ounces
Head Cast ~1 – 2 pounds
Half Torso Front ~.75 pounds
Full Torso Front ~1.5 pounds

In a bucket mold, Alginate is mixed and placed into a container (bucket). The body part (a hand is a perfect example) is pushed down into the Alginate to make the mold. The bucket holds the Alginate and keeps it in place.

In a Bucket Mold, the Alginate fills the space between the hand and the inside of the bucket so the smaller the bucket, the less Alginate you will require. The bucket must be big enough so the body part doesn’t touch the inside or the bottom, but not overly large. (Just imagine how much Alginate you’d need if you did a hand casting in a bathtub.)

The easiest way to calculate your Alginate requirement is:

  • Fill the bucket with water
  • Put the body part in - allowing the excess water to flow out
  • Measure the remaining water
  • Since a pint of water weighs almost exactly 1 pound, for every pint (16 fl.oz) water you have left over, use 1/4 pound of alginate. That’s our 4:1 ratio.

Bigger jobs generally require more working time.

The Layup Mold

Layup mold Alginate requirements are based on square inches of area to be covered by the Alginate. Since Alginate is usually applied about 1/4 inch thick, some simple calculations should give you what you need.

1 pound of alginate mixed 4:1 (water to powder by weight) has a total volume of approximately 137 cubic inches. Mixed at 3.75:1 (our “thick mix”) that figure drops to about 128 cubic inches per pound.

Now, since we are only applying the alginate 1/4 inch thick, we multiply those figures by 4 and get an approximate surface area coverage, per pound, of:

548 square inches at 4:1
512 square inches at 3.75:1

An approximate measurement of the area to be covered can be done by measuring the height and width of the area (in inches) and multiplying them together.

A face is generally a circle about 12 inches across. That give us a figure of 113 square inches, so a face can easily be done with just 1/4 pound of Alginate (actually 3 ounces is generally enough).

Water/Powder Ratios

General alginate mixing instructions call for a 4:1 (water weight to powder weight) mixing ratio. This is great for 'Bucket Molds':

4 oz. alginate to 1 pound water (~1 pint)
1 pound alginate to 4 pounds water (~2 quarts)
2 pounds alginate to 8 pounds water (~1 gallon)

For 'Layup Molds' like faces, heads, torsos, etc. we recommend a “thick mix” of 3.75:1 by weight:

4 oz. alginate to 14 ounces water (14 fl.oz)
1 pound alginate to 3.5 pounds water (~3.5 pints)
2 pounds alginate to 7 pounds water (~3.5 quarts)
The reason we don’t recommend measuring out your alginate by “volume” is that alginate is a “compressible powder”. This means that alginate can be at a wide range of densities at any one time. In fact, fully compressed alginate weighs almost 40% more than fully fluffed alginate.

If you MUST measure alginate by volume, use 4 parts alginate to 5 parts water. This is a decent approximation. Remember this is for volumetric measurements only and will not be as accurate as measuring by weight.

Once you get used to working with alginate and know what you like, you can portion your alginate however you want. We have several customers who mix their alginate at 7:1 (very, very thin). It works for them so we don’t question them.

What happens when alginate is mixed very thinly is:

  • The setting time is extended
  • The setting strength is decreased
  • The shrinkage rate after it sets is greatly increased


Mixing alginate thinner can save you money.

Alginate Setting Time

The three number sequence refers to the setting time at a particular water temperature. (For example, under ideal conditions, 590-IBG alginate will set in about 5 minutes when using 90°F water.) GenesisV sets in about 5:30 with 70°F water and its brother, GenesisX will set in about 10 minutes under the same circumstances.

  • 270-D……2 minute set w/70°F water
  • 370-SD……3 minute set w/70°F water
  • 380-CC……3 minute set w/80°F water
  • 390-S2……3 minute set w/90°F water
  • 570-PGV……5 minute set w/70°F water
  • 590-IBG……5 minute set w/90°F water
  • GenesisV……5 min. 30 sec. set w/70°F water
  • 880-B……8 minute set w/80°F water
  • GenesisX……10 minute set w/70°F water

Of course we test our alginates during manufacturing so they perform to these standards (+/-5%). We test under controlled conditions (low humidity and 70°F air temperature with purified water).

Rarely will you have the luxury of using our products under such conditions. Here is a list of factors that can affect the alginate setting time (in order of the magnitude of their effect):

Water Temperature: Warmer water will speed the setting time of alginate and cooler water will slow it.

Mixing Ratio: Thicker alginate sets faster, thinner sets slower - all else being equal.

Air Temperature: Air temperature will affect how quickly the alginate will cool during and after mixing- same “warmer/faster, cooler slower” effect, however layup molds will be much more affected because of the larger “surface area to volume” ratio. Spread thin, it will be more affected by air temperature and evaporation than a bucket mold.

Humidity: Since evaporation off the surface of the alginate will cool the mass, lower humidity will increase evaporation, cooling the mix and slowing the set. Higher humidity will lessen evaporation, allowing the alginate to stay warmer which will shorten the setting time.

Chemicals in the water: In general, our recommendation is, “If you feel fine drinking your tap water, then its OK to use to mix your alginate”. If you filter your drinking water or there are noticeable odors from your tap water, best to use bottled water. The cheap stuff is fine. We have no idea what effects your tap water might have on the setting time or any other working property of your alginate. 

Pouring Time

People ask us all the time, “How long before I have to pour my alginate mold?”
The answer is: “Right away, if you’re looking for the best results”.

Unfortunately, an alginate mold doesn’t last very long. It begins shrinking almost immediately. At first the shrinkage is very small, but it is cumulative and non-reversable. We’re always looking for our customers to get the best possible results, which is why we say “GET IT POURED !!!”

We recommend that you consider the “mold-making part” and the “pour-up part” to be one procedure instead of two.

If you’re still reading this, you might want to know why alginate shrinks and what influences shrinkage. Here you go.

Alginate is, on a microscopic level, just like a wet sponge. There is really nothing holding the water IN the system. Water evaporates off the surface and is immediately replaced by the water just under the surface, etc., etc., etc. This is knowns as “transpiration”. Another way water leaves the system is through syneresis. Imagine the mass of alginate being a water balloon, with millions of very, very small holes in it. Just the pressure of the weight of the alginate mass “pushes” water out through these holes.

The upshot of all this is that the water leaks out of the “sponge” and the alginate mass inevitably shrinks. It will eventually shrink to about half its original size and be as hard as a rock. Shrinkage also does not happen uniformly. thinner sections shrink faster than thicker ones, so distortion is inevitable. Keeping the alginate in a 100% humidity environment. Covering the surface of the alginate with a wet towel dramatically slows the evaporation, but not the syneresis.

Actually, evaporation is much more pronounced in “Layup Molds” than in “Bucket Molds” because they have a much higher “surface area to volume” ratio. These are exposing a much higher percentage of their volume to the air, so the evaporative effect is greater than it would be for a “Bucket Mold”.

Is Alginate Safe?

Alginate is incredibly safe. Dental alginate, essentially the same as prosthetic alginate, has been used hundreds of millions of times IN people’s mouths by dentists all over the world during the past 70 years. To our knowledge, there has never been a verified instance of any allergic reaction to alginate. Neither have there been any verified cases of long-term injury from the use of alginate, even when used on a daily basis in a dental office.

Some of our competitors have developed alginate formulas that they claim to be “Silica Free”. They claim some huge SAFETY advantages with their formulas. Never do they claim that their Silica Free alginates are better than more traditional formulas. 

Alginate is a fairly complex blend of between 7 and 15 ingredients. One or more of these are “Functional Fillers”. They add nothing to the chemical reactions going on inside the alginate, but they do help regulate the mixing, working and setting properties. Without these fillers, alginate would be a useless mess.

The most commonly used of these fillers is diatomaceous earth (DE). DE is comprised of the microscopic shells and skeletal remains of miniscule animals that lived in prehistoric shallow seas. These shells have extremely complex shapes and lots of holes in and through them. They absorb a great deal of water.

In the processing of DE (from a mined product that contains lots of other dirts and organic materials), it is “Calcined” or heated to a very high temperature. This burns off the organic materials and helps purify the DE. In this heating process, a small percentage (less than 1%) of the DE melts and recrystallizes. These crystals are found in any dust that is generated by the DE in an alginate formula. OSHA and other safety organizations have found that breathing large amounts of “Crystalline Silica” over an extended period of time can cause lung cancer.

This is why most alginate manufacturers (including us) have developed “Dust Free” Formulas that reduce the amount of dust produced by alginate by over 95%.

This combined with the fact that the DE is one of the LEAST dusty components in our alginate formulas and the fact that most people do not spend their lives working in and around alginate- makes modern alginate very, very safe even without the “Silica Free” formula.

To our knowledge, no one has ever contracted ANY lung disease from working with alginate- and it is much safer today than it was just 10 years ago.