Alginate "Setting" (What does this mean?)

Alginate "Setting" (What does this mean?)

Posted by Chris Baker (Dr. Alginate) on Apr 2nd 2018

How Much do You Know about Alginate Setting?

Even people who use alginate a lot are often unclear about what makes alginate set and how other factors can affect the setting time. I thought I’d share with you what I know.

Alginate is pretty simple. The main ingredients are Sodium and/or Potassium Alginate. These powders are derived from the stems of certain types of seaweed. The seaweed is dried, pulverized, treated and rinsed to leave only the organic polymers. Polymers are long rope-like molecules that are flexible, but very strong when you pull on them. They’re what keeps seaweed from breaking when it is subjected to the turbulence of the ocean.

Anyway, think of these polymers as little pieces of string. When mixed with water, these pieces of string are free to move about the string/water solution. The setting reaction in alginate effectively GLUES these pieces of string together wherever they touch. String A is glued to string B where they touch. B is also glued to C, D, E, F…. wherever it touches them. What this produces is a three-dimensional, flexible solid. Of course all this is taking place on a microscopic level.

It is actually Calcium ions released from Calcium Sulfate that act as the glue between polymers. Various other chemicals we add to the alginate powder determine the speed at which the setting takes place and other working properties of our alginates. This 3-dimensional matrix of polymers do not chemically bind the water into the system. The water is only mechanically retained and it is free to slowly leak out- which causes the inevitable shrinking of an alginate mold (but that’s another discussion).

Factors that affect setting time…

1) The most important factor is water temperature. The chemicals in an alginate mixture perform their jobs as they “go into solution” (dissolve into the water) and mix with the other chemicals. You will have noticed that sugar dissolves faster in hot water than it does in cold water- and the same thing is going on here. Faster dissolving, faster reacting, faster setting. Simple.

2) The second biggest factor in the setting time of alginate is “how much water you use”. Alginate can be mixed thick or thin, or anywhere in between. Our standard mixing ratio is 4:1 (water weight to powder weight). At that ratio, our alginates conform pretty closely with their targets. For example at 4:1, our 570-PGV alginate will set in 5 minutes (+/- 15 seconds) when mixed with 70°F water in a 70°F room. If alginate is mixed thinner than 4:1 (more water), it will slow the setting time, because you have diluted the chemicals in the alginate mix that are responsible for starting the setting reaction. Also, water is usually quite a neutral pH and some of the reactions in alginate are pH driven so too much water will delay the set. The opposite is true when mixing alginate thicker than 4:1- for the same reasons.

3) The third factor is room temperature. A cold room will rob the alginate mixture of its heat during both mixing and application. Colder alginate, as seen in #1 above, will slow the setting time. The opposite is true of a very warm environment. It will speed the setting of the alginate for the same reason. Smaller quantities of alginate will be MORE affected by room temperature than larger quantities.

4) The fourth factor is the type of bowl/bucket you mix your alginate in. Metal is a good conductor of heat. Plastic or rubber are better insulators. Mix alginate in a metal bowl and it will be cooler at the end of the mix time than the very same mixture done in a plastic bowl. Since water temperature has a big effect on setting time, so will the type of container you mix in. Keep that in mind.

5) The fifth factor is humidity. A 100°F day in Phoenix doesn’t feel nearly as hot as a 100°F day in Atlanta, because of the difference in humidity. In Arizona, sweat evaporates quickly and leaves you feeling much cooler than you would in Atlanta where the humidity is likely to be quite high. Same thing holds for alginate. On a very humid day, there will be much less evaporation off the surface of the alginate mixture so it won’t cool as much during mixing as it does on a dry day. According to #1 above, a warmer alginate mixture will set faster than a cooler one (all else being equal), so alginate is going to set slower in Phoenix than it does in Atlanta. Keep in mind, air conditioning also removes a lot of the moisture from the air so it will be about the same humidity indoors in Atlanta as it will be in Phoenix, but not quite.

6) The compound we use to slow the setting time of alginate is called Tetrasodium Pyrophosphate. Sounds like rocket fuel, right? Its actually a pretty harmless chemical related to STP which is commonly used to wash windows. If you have a project that you need a REALLY long setting alginate for, we can send you some. Just call.

That’s all I know on this topic. I hope that those of you who are freaks and still reading this post have learned a little something about alginate. As always, if you have further questions I’m pretty much always available to answer questions via email, chris.baker@accu-cast.us, or by phone (855) 773-0460.