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Alginate Impression Material


Alginate impression material has been used by dentists for decades to make dental molds of patient’s teeth. Because it is a skin safe and hypoallergenic material – safe even for use on the youngest child, it was also used to make impressions of limbs and other body part to fit prosthetics. Soon artists and sculptors began experimenting with this mold making compound. They discovered that they could make molds of their model's posing (click here) that they could use in create their sculpture pieces well after their model had left. Because unlike posing models, inanimate objects never tire. Soon artists began creating life casts for the single purpose of casting art pieces. But dental alginate had a few drawbacks, foremost was that it cured in just 2-3 minutes, making its use limited to smaller body molds. As there wasn’t enough time to create larger pieces before the alginate set. Suppliers were able to modify formulas for these artistic demands, to create what is known as prosthetic grade alginate. In this article we reveal its strengths, weaknesses and instruct on how to best use prosthetic grade alginate you to create perfect lifecasting molds.

Giant Kelp at Birch Aquarium, San Diego CA

Figure 1. Boy observing giant kelp (Macrocystis pyrifera) at the Birch Aquarium, San Diego, CA. Kelp can grow 2 1/2 feet per day and when full grown can be as high as 130 feet. Kelp is harvested to extract its algin used in the production of skin-safe impression mold material.

A Brief History

In 1883, a UK pharmacist, E.C. Stanford developed the compound he called, alginic acid or algin. He found that this compound was present in most seaweeds as a complex salt of sodium, potassium, calcium and magnesium. This is important to know as in mixing today's prosthetic grade impression material formulations you do not want to add more of these salts as it will impede the curing times of the impression material. Such salts are dissolved in water and create what we know as “hard water.

The exact combination of algic acid varies with algal species. But the commercial production of algin salts did not begin until 1929 through the Kelco Company, which was located in California. It is now a division of Merk. Since then the industry has grown with major producers being in the United States, the United Kingdom, Norway, Canada, France, Japan and China. (click here).

The US Navy is credited for developing a compound we know today as dental impression material for the dental industry. It was developed as a substitute for agar agar. At the time, Japan was the only source for agar agar, but the hostilities between the two countries cut the United States off from its supply.

For years, algin salts have been a staple in pharmaceutical applications. As a hydrocolloid of vegetable origin, occurring as a structural component in marine brown algae, it gives strength and flexibility to the plant. More than twenty stages of processing are required to extract algin (also known as alginate) from its natural seaweed source.

A Highly Useful Product

The algin market is very large. By 2025 it is expect to reach USD $923.8 million (Grand View Research, Inc. report) That is because its uses finds broad applications in the pharmaceutical sector and acts as an emulsion stabilizer, thickening agent, disintegrant, and film-forming agent for tablets.

Today more than 25,000 tons of alginic acid is extracted each year worldwide. In the USA the giant kelp, Macrocystis pyrifera is used. It is harvested from large offshore beds off the coast of California and Mexico.

Some 120,000-ton wet weight is harvested each year using ships equipped with cutting machinery. Macrocystis has the distinction of being the largest seaweed in the world. The largest attached plant recorded was 65 meters long. The plants are capable of growing up to 50 cm per day.

harvesting kelp

Figure 2. Kelp harvesting in California waters. This is the first step in the production of alginate.

Alginates find their uses in a variety of industries, such as pharmaceutical and cosmetics, but one of the most important consumers are in textiles (50%) and food (30%). In the food industry it is used as a stabilizer of ice cream, making the product fine, smooth, tasty and with a better melt-resistance. In the bakery industry it is used in making the bread lighter and softer and to resist drying out. It is used in the production of dried noodles for added strength and to increase the yield of the finished product.

In the confectionary industry it is used in the production of chocolate, milk sugar, soft agar sugar and other high-quality sweets. When the appropriate algin type is added, it can raise the melting point of the product and improve its toughness and reduce its stickiness. In the beverage industry it is used to reduce precipitate. It is also used for coating for a preservative for fruit, fish, meat and other foods.

It is an important ingredient in the textile industry as well. A small quantity of it is used to replace a large quantity of starch. It is an important additive in the printing and dyeing industry, used as a thickening agent. It has a number of uses in the medical industry, too. This special material is used for intestinal disease administration, dental impression materials, a radiography agent for barium meals, medicines for high blood pressure and gastric diseases.

Algin Extraction and Processing - Producing Alginic Acid

Algins can be obtained from fresh (wet) or dry kelp. However, the process of extraction of the pure material is difficult and firms do not readily reveal their methods. In a typical process, insoluble the salts in the seaweed are converted with an alkali which is readily soluble in water. Further dilution with fresh water causes the algal cell structure to break down and release sodium alginate into solution. Sodium and potassium alginate are the prime ingredients used to produce mold making impression material and is what give this material its name.

Insoluble seaweed particles are then separated from the solution by standard solid-liquid clarification through the use of a centrifuge. The algins are then recovered from the clarified sodium solution by several alternate techniques. This would include precipitation with calcium chloride or sulfuric acid, electrolysis or direct drying. The alginic acid is neutralized with different bases, then dried and milled to produce the final product.

The Formula Known as Dental Impression Material

Sodium, potassium, or triethanolamine alginate is the main ingredient in this irreversible hydrocolloid known as dental impression and prosthetic grade cream. These products are classified as irreversible hydrocolloid because it consists of particles of a gelatinous (colloidal) state in water (hydro) and irreversible because once it has jelled it cannot be returned to a liquid solution. Characterized according to its different physio-chemical properties as gelling agents, various mix ratios create impression materials with different qualities.

Ross ribbon mixer

Figure 3. Stainless steel ribbon blender used to mix 500-pound batches of prosthetic grade impression material. The various ingredients are added from the top. After thorough mixing, the blended impression materials drop out of the bottom of the blender.

Diatomaceous earth (DE) consists of the sedimentary deposits formed from the skeletal remains of a class of algae diatoms (Bacillariophyceae) that occur in both fresh and salt water and in soil. These remains form a diatomite, almost pure silica that is ground into an abrasive powder. It is primarily silicon dioxide (glass, sand).

A prosthetic grade impression material formula contains a sodium or potassium alginate at a level of up to 15-percent, a calcium source used as a reaction retarder at the level of about 8-percent (such as tetra sodium pyrophosphate) pH modifiers (such as magnesium oxide or potassium fluoritanate --acidic) at 0.5 – 1-percent and a filler such as diatomaceous earth, about 75-percent for stability.

Formulas for lifecasting (prosthetic grade) impression materials vary greatly. Typically, the formulas used by body casting artists and mold makers are manufactured to dental grade specifications but modified to adjust for longer set times and shrink rates as well as added tear strengths. For beginners it is helpful to review alginate selection advice available from YouTube and on lifecasting websites such as this one.

Prosthetic Grade Impression Material Use in Lifecasting Artistry

Because of its history of safety in its use as a dental impression material, as a prosthetic limb mold making material, aligns were discovered by early lifecasting artists to be an ideal choice for artists, model maker and sculptors who desired to make mold directly from the human body. Prior to its use, life molds were often made with gypsum plaster – sometimes causing injuries due the heat generated during the curing stage. But these prosthetic grade creams were a water-based material, were completely safe and generated no heat. In fact, it was actually so comfortable that it is used in facial treatments at day spas.


One of the more interesting characteristics of prosthetic grade cream is that it does not adhere to itself once set. That same characteristic is helpful in life cast mold making, as it, like Teflon, doesn’t stick to anything (except the stainless steel of a JiffyMixer). However, it can be made to stick to itself by increasing the pH of the cured surface. This is easily accomplished by painting its surface with a mixture of baking soda and water. Once applied, another coat of prosthetic grade alginate can be added which now will adhere to the previous coat.
Prosthetic grade cream set time can be adjusted by varying water temperature. The warmer the mix water or the thicker the mix, the faster the curing. The converse is also true. The cooler the water, or the thinner the mix the slower the curing. However, since life casting is the practice of applying mold material to the skin of a live person, cold mold material creates both goose bumps and model discomfort. Newer formulations have been developed specifically for lifecasting mold making that allow for set times up to nine minutes or more using water temperatures approaching body temperature.

It’s All About the Water Quality

Manufacturers carefully control formulation blending. Its quality control departments test all batches of its prosthetic grade impression materials before packaging. The manufacturer uses de-ionized water for quality testing. De-ionized water is also known as distilled or soft water. That is because its formulas are sensitive to over mineralization such as those minerals found in hard water, for example untreated well water.

Excess minerals will cause a lumpy mix somewhat akin to cottage cheese. Lumpy impression material will severely mar the surface of any life casting mold wasting time and money. Extremely hard water will prevent impression material formulas to cure altogether.

Usually tap water from a municipal water source is good enough to create a smooth mix. But that is not a hard and fast rule. Experience lifecasters will always test a small batch before attempting a studio mold. When in doubt use bottled water – or even better distilled water. Poor water quality has ruined more casting sessions, so lifecaster beware. The manufacturer controls the formula quality. But, it is the user's responsibility to control the water quality.

Ideal proportions of water-to-impression material varies from manufacturer to manufacturer and with the user’s personal preference. The mixing proportions suggested by the manufacturer represents what it feels yield the optimum working consistency. However, we would suggest that in order to determine what proportions will produce the best consistency, the artist should experiment.

For example, a thicker consistency would be needed to make a mold in the horizontal position to prevent excess running and dripping. Whereas a thinner consistency preferable be desired for container molding.

Alginate Weaknesses as a Mold Making Material

With every benefit to a product or service there are often weaknesses. Prosthetic grade impression material has a few. It is known as a waste mold as most impression material molds can only be used once. It cannot be used for multiple castings for several reasons. Most formulations are not strong enough to resist tearing for multiple casting. Although EnvironMolds, ArtMolds, FiberGel product can survive a few castings due its excellent tear strength resulting in fiber contained in the product.

Another characteristic of prosthetic grade impression material formulas for mold making is that the mold will begin to shrink soon after it sets, losing detail in a few hours and losing up to a third of its size in twenty-four hours. Though placing the mold in a sealed plastic bag with a wet sponge or rag will delay shrinkage and extend casting time.

The mold maker is also limited to the type of material that may be cast in an prosthetic grade cream mold. While plasters, silicone rubbers and waxes are the perfect casting companions, resins of all kinds are not. That is because polyester and polyurethane resins react poorly to the moisture in the mold and will foam up on contact.

artmolds alginate line

Figure 4. The ArtMolds line of prosthetic grade cream products. From left to right. MoldGel Regular Set (4-5 minutes setting) SILFREE (contains no silica); MoldGel SloSet Set (7-8 minutes setting) SILFREE (contains no silica); Hollywood Impressions (2-3 minutes setting) SILFREE (contains no silica); FiberGel Highly Tear Resistant (5-6 minutes setting); Algislo Alginate Bonder, Cleaner, Retarder; MoldGel Regular Set (4-5 minutes setting) Traditonal Formula; MoldGel SloSet Set (7-8 minutes setting) Traditonal Formula; Hollywood Impressions (2-3 minutes setting) Traditonal Formula.

Personal Experience by a Master Lifecasting

The late Dave Parvin, fine art sculptor, lifecasting teacher and frequent contributor to the Art Casting Journal, describes his experiences.

“Years ago, I developed a solution that I have used with great success and taught in all my workshops – I felt then it was perfectly adequate. I would apply a layer of alginate that was just the right consistency and temperature to stay in place and be comfortable and allow enough time for a flawless second layer into which I would embed the fuzzy material. The trick is to paint over the first layer with a mild base such as solution of baking soda and water without which the layers would not bond together. With this technique I have been able to do far more complicated castings than would be otherwise possible. The only downside was the extra time cost required for the second layer.”

“Particular brands are for the most part simply a matter of personal choice since most are pretty much interchangeable. However, most alginates have the same characteristic in that they exist in either a liquid or a solid state with a very short transition stage. For simple casts such as hands or face without hair, ears and shoulders, this poses no problem.”

“But as one progresses to more complicated castings, it becomes very difficult to apply and embed a layer of fuzzy material into the molding material to hold a plaster shell mold fast before it sets-up. While either increasing the ratio of water or lowering the temperature of the water can extend set time, neither of these solutions is adequate. Too runny and the it will not stay in place and too cold is not comfortable for the subject.”

“There is however, another solution. MoldGel Sloset from ArtMolds is uniquely different from any other alginate I am aware of in three ways. The first is that it has the slowest setting time of any I tried, 8 to 9 minutes in warm water. Secondly it is more thixotropic (i.e. it stays in place without running off the model). Thirdly, the transition from a liquid to a solid is far more gradual. The result is that one can apply an adequately thick layer that stays in place even in undercuts such as below the ears, breasts, chins, etc. and on small pointy places such as noses, nipples, finger tips, etc. This eliminates, for the most part, the need to keep applying additional material to hard-to-cover areas. This time saved along with the slower setting time allows the caster to apply the fuzz before the it gels.”

“In addition, the more gradual gelling further prevents runoffs and indicates to the caster how much time is remaining. Eliminating the second layer reduces the overall time required for the process making it easier on the model. Because there is less runoff, less is wasted and the need for a second layer is eliminated.

We also recommend high strength FiberGel E F/X Grade cream

About EnvironMolds

In 2018, Environmolds celebrated its 20th year offering environmentally and user safe mold making and casting materials, including its line of ArtMolds brand products sold in fine art and hobby stores worldwide. The company and its website,, offers a library of how-to articles and videos to help both the novice and more experienced artist and mold maker with materials that allow you to reproduce almost anything including our variety of prosthetic grade creams, latex rubber, various silicone rubbers, polyurethane resins, gypsum plasters and related mold making and casting materials and tools.

Why Buy from Us

EnvironMold with two decades of manufacturing has the experience, knowledge and capacity to help you successfully complete your reproduction projections. You don't have to make a purchase to have our help, as we having been aiding artists and sculptors with our hot line available 24/7. As to prosthetic grade cream purchases, combined with MoldGel’s lower price than most other brands, one’s mold making cost is reduced by as much as 50-percent. For the above reasons MoldGel SloSet is my first choice for anything except infants’ hands.

Now that you know all about alginate, it is advisable that the lifecasting artist experiment with a variety of brands. Each brand will provide unique characteristics. No matter which brand one settles on, one can count on a product category that will provide excellent reproduction and the certainty of safety with their model. We believe in the end you will return to making the ArtMolds brand your brand, too.

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