Wow, there is a lot of information out there trying to define chalkware. If I may offer my interpretation on it below.
Let’s start with all the names of the material that I have stumbled across:
Gypsum Plaster
A mixture of materials which includes the commonly found hydrated calcium sulfate mineral, gypsum[1]. It is typically mixed with a number of hardeners or retarders and water. The result of mixing with water hardens the mixture over time.
To improve the strength of the gypsum plaster[2], it can be further mixed with more chemicals (such as borate, alum or alkali sulfate and baked under high temperature to completely dehydrate, thus producing “plaster”. This technique has been around for centuries, as far back as 7500BC[3].
This type of materials was used by Bosson to develop their wall masks which were made in the 1960s, 70s and 80s. In the excellent book by Robert E. Davis[5] the technique of production is described in the following stages:
- Start with a clay model. This allowed the producers to make very intricate designs
- Prepare a case mold. As the book notes, this typically meant the original clay model was destroyed
- A “plaster cast” can then be made from the mold
- The sculptor could then add further detail to the plaster cast
- Drying of the piece takes place over a number of days under controlled temperature and humidity conditions
In latter years, Ray Bosson used polyvinylchloride (PVC) to coat the piece, thus removing any minor surface irregularities in the master.
Plaster of Paris
So Plaster of Paris is gypsum-based containing a calcium sulfate hemihydrate. Ornamental plaster based on plaster of Paris started in the 1830s[4] (maybe earlier in other countries?) as the molded plaster cornices fund between the wall and ceiling.
Plaster of Paris is prepared by heating calcium sulfate dihydrate, or gypsum, to 120–180 °C (248–356 °F).
You may also know of its popular properties in the medical industry. It was first used in mass casualties in the 1850s[6].
Chalkware
So where does the phrase “chalkware” come in. Well the main difference between Plaster of Paris and chalkware is this;
- Plaster of Paris – Gypsum is that hydrated crystallised mineral from above, made from Calcium Sulphate (CaSO4)
- Chalk – A mineral deposit which is made from Calcium Carbonate (CaCO3) in the form of a sediment made up of millions of calcareous skeletons of a microfossil called a coccolith
See, that extra ‘O’ makes all the difference!
And it means that it isn’t really “chalkware”, but the name seems better than “gypsumware”…
References:
[1] – https://www.britannica.com/science/gypsum
[2] – https://www.britannica.com/technology/gypsum-plaster
[5] – Davis, Robert E., ed. (1982) The Imagical World of Bossons. 1st ed. W. H. Wolfe Associates
1950s vintage chalkware statues 