PrairieGirl

Prairie Green Glaze Variations

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I have become an avid collector of Prairie Green Frankoma, and I am hoping someone can answer a question about the glaze variations among my pieces. All my pieces are Prairie Green on red clay, but some are more glossy and others very matte. The glossier pieces also tend to have a darker brown background than the more flat pieces. I have also noticed a much more true green color on my Lazybones dishes than on some of the more decorative and glossy pieces that are a little more bluish in their green (but definitely not blue). I am interested in learning anything you might know about these variations. Thanks for your help.

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Since my collection contains more Ada clay than Sapulpa (red) clay pieces, I don't know as much about the Sapulpa clay pieces. However, I can tell you these things: I have heard Prairie Green pieces that have a lot of brown in them referred to as "over-glazed." I'm not sure what that means. Also, Frankoma began using a different type of rutile glaze in 1970. This definitely had an effect on the finishes. Your glossy pieces may be made with the post-1970 glaze, and the more matte pieces would have been made with the pre-1970 rutile glaze. (From what I understand, "rutile" is a chemical or perhaps a mineral element that is widely used in ceramic and pottery glazes.) Somebody help me out here.

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Since my collection contains more Ada clay than Sapulpa (red) clay pieces, I don't know as much about the Sapulpa clay pieces. However, I can tell you these things: I have heard Prairie Green pieces that have a lot of brown in them referred to as "over-glazed." I'm not sure what that means. Also, Frankoma began using a different type of rutile glaze in 1970. This definitely had an effect on the finishes. Your glossy pieces may be made with the post-1970 glaze, and the more matte pieces would have been made with the pre-1970 rutile glaze. (From what I understand, "rutile" is a chemical or perhaps a mineral element that is widely used in ceramic and pottery glazes.) Somebody help me out here.

While the age (and type of rutile used) will have some impact, your first suggestion is probably the more relevant. The way rutile glazes work is by taking advantage of different glaze thicknesses. Even the best glazer in the world will not have a uniform glaze over the entire piece; there will be some thick spots and some thin spots. The thick spots will be brown and more glossy and the thin spots will be green and matte (in the case of prairie green). The same thing applies with other rutile glazes (brown satin, woodland moss, peach glow, etc). For the non-rutile glazes, the thickness will simply change the opacity and color. Thus, if you look at a white piece, the thin areas (typically on the raised parts of the item) will let some of the clay show through while the thicker areas (such as in a 'valley') will be solid, opaque white. During the years when only a few people were glazing Frankoma, it was possible to tell which pieces were glazed by which person depending on the final appearance of the glaze because some people are just naturally inclined to use heavier glaze loads.

The observation that some of the pieces have a different hue (more blue) than the others is most likely due to changes in the formulation. Glaze materials are not constant and older materials are constantly being phased out and replaced by new ones. Even when the supplier is sending nominally the same material, there are real, observable variations in particle size and chemical composition which can lead to a slightly different hue. This is especially noticeable in the rutile glazes--brown satin, desert gold, and prairie green appear to be the most strongly affected.

To answer your other question: rutile is a mineral (also a chemical) containing mostly titanium dioxide. It actually has some very interesting characteristics: http://www.galleries.com/Minerals/Oxides/RUTILE/RUTILE.htm. There are variations in levels (and type) of impurities, particle size, and so on. Rutile is used to add opaqueness to the glaze (it falls into the categories of opacifiers). For that matter, titanium dioxide is also widely used as an opacifier in paint, but because of the demand for more consistent, reproducible color, paint titanium dioxide is usually synthetic. In any event, the rutile dissolves in the molten glaze at very high temperatures, but then as the piece cools, it can precipitate out. Whether it precipitates out or not depends on the thickness of the glaze, the cooling rate, and other constituents of the glaze. It's really not possible to give a simple explanation of how it works, but if the glaze crystallizes in large crystals, you get the green, matte portions of the glaze. If it doesn't precipitate (or precipitates in crystals smaller than the wavelength of light), you get the smooth glossy appearance. Think of it this way: snow and ice are frozen water. Under some conditions, you get large, fluffy snow flakes. Other conditions give you tiny, dense snowflakes. Still others give you freezing rain, sleet, hail, and so on. Same material, but different conditions during its formation change the form of the crystals. The same is true of rutile glazes- the only difference is that different parts of the same piece see different conditions. The same effect that causes the 'rutile effect' in pottery is what is responsible for the appearance of tiger's eye or star sapphire gems.

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