Perlite

[George O'Hanlon]

We have just added another interesting mineral to our array of extenders and fillers for grounds and paints: perlite.

Perlite is an amorphous volcanic glass that has a relatively high water content, typically formed by the hydration of obsidian. It occurs naturally and has the unusual property of greatly expanding when heated sufficiently. It is an industrial mineral and a commercial product useful for its light weight after processing.

The processed mineral is a light, fluffy white powder that has some interesting properties when added to plaster or chalk grounds. We are using it combined with plaster and Portland cement to make lightweight fresco panels.

Properties
When crude perlite is heated rapidly to temperatures above 870° C (1,600° F), it pops in a manner similar to popcorn creating countless tiny bubbles in the heat softened glassy particles. The combined water vaporizes and escapes, causing the material to expand 7 to 16 times its original volume. Unprocessed perlite has a bulk density around 1100 kg/m3 (1.1 g/cm3), while expanded perlite typically has a bulk density of about 30–150 kg/m3, so it is much lighter in weight per volume. It is these tiny glass-sealed bubbles that account for the amazing light weight and other exceptional physical properties of expanded perlite.

The expansion process also creates one of perlite's most distinguishing characteristics: its white color. While the crude perlite rock may range from transparent to light gray to glossy black, the color of expanded perlite ranges from snowy white to grayish white.

Source
Our perlite comes from the Nu-Alexite and Socorro perlite deposits located in New Mexico, U.S. The ore is formed by magmatic processes, usually in "domes," that over geologic time periods entrap water into the matrix. The mechanism through which this occurs is not completely understood, however, it is this entrapped water which gives perlite ore it's unique ability to expand. The Socorro deposit has proven reserves of over 10 million tons and is unique in that it is believed to be the deepest known perlite deposit in the world. These characteristics give the processed Socorro ore unmatched quality and consistency. The perlite ore is surface mined and is usually light gray in color. All perlite ore contains low crystalline silica contents in comparison with other industrial minerals.

Uses
Due to its low density and relatively low price, many commercial applications for perlite have developed. In the construction and manufacturing fields, it is used in lightweight plasters and mortars, insulation and ceiling tiles.

Perlite is an excellent filter aid. It is used extensively as an alternative to diatomaceous earth. The popularity of perlite usage in this application is growing considerably worldwide.

Although applications of perlite in art are uncommon, everyday it is being used more and more in such applications as plaster, ceramics, foundry casting and paint.

Use in Paint
Perlite can be added to paint and painting mediums, especially water-based, to add bulk. For example, add perlite to calcite mediums to increase the bulk of the medium or paint for thick impastos. It can also be used to prevent settling of pigments and heavy extenders. It can also be used to add texture or build the consistency (thicken) of paint.

Use in Grounds
A novel use of perlite is in painting grounds. Add an amount to a traditional gesso or chalk ground to increase the bulk and decrease its weight.

[Steve Ladd]

George-
I think perlite was the light-weight filler in that product called Structolite that we talked about a while back in regards to fresco panels. Did you ever come across any of the Structo-stuff? Your panel mixtures are no doubt better with just the perlite and cement than with the other material. I think structolite had a bunch of other ingredients, maybe gypsum or plaster , etc. that made for a weaker mortar. But it sounds like you expanded the uses of perlite greatly so it can be used even in oil mediums. Interesting possiblities for large textured paintings, like blending it with Velazques Medium to make a kind of fluffy impasto.

[George O'Hanlon]

Yes, perlite is an ingredient in Structolite, besides plaster.

[J.A. Borgesse]

George,
I recently purchased your barite powder for experimentation. I have not worked with it yet as I am finishing up my tests and experiments with the Epoxide Oil and your calcite based mediums. what advantages would the perlite have over calcite and/or barite when employed in an impasto medium? Also, can perlite be combined with the aforementioned powders in an impasto medium?

JAB
website
http://www.jaborgesse.com/

[George O'Hanlon]

Originally posted by J.A. Borgesse:
...what advantages would the perlite have over calcite and/or barite when employed in an impasto medium? Also, can perlite be combined with the aforementioned powders in an impasto medium?

I do not look at perlite (or another mineral) as having advantages over any others, but it has certain qualities that when combined can offer advantages. It can be combined with other extenders in oil paint to add texture, bulk and transparency.

[J.A. Borgesse]

George,
Would the low density and light weight of perlite generally create a weaker impasto mass or film than barite which is dense and heavy by comparison? I mean when used seperately as a single ingredient mixed with oil to create an impasto medium.
JAB
website:
http://www.jaborgesse.com/

[George O'Hanlon]

Silicates do not form good paint films when used alone, so an extender composed largely of silica such as perlite or quartz would not be good to use alone. However, most artists would combine this with a color, and hence it would become a minor component. The lightweight does not contribute or detract from the strength of the film, and in fact in small proportions is used to improve film strength.