Just the words “copper red” will capture the attention of most potters working at stoneware temperatures. A lot has been written about copper red glazes since the 1970s, but much of the information presented is contradictory. Because it is such a hard-to-perfect glaze, it would seem logical that a precise and complicated glaze recipe would be required to produce good results. But I have found that more complicated does not always mean better.
For some reason, most copper red glaze recipes I have seen include small amounts of zinc oxide and barium carbonate, yet I have often wondered what purpose they served. With help and encouragement via e-mail from glaze consultant Ron Roy, I decided to test five variations of my best and most reliable recipe, to see what would happen if zinc and barium were omitted. Although I had already modified the recipe over the years, mostly in an attempt to reduce crazing on my clay body, this glaze, known as “Chun Red,” has been passed among potters for several decades.
When I first started experimenting with copper red glazes in the 1970s, I referred to Herbert Sanders’ book Glazes for Special Effects. It included dozens of recipes, and every single one used zinc and barium. Nowhere in the text did Sanders give an indication as to why these ingredients were important.
Likewise, in Clay and Glazes for the Potter, in the section on reduction glazes, Daniel Rhodes included a “typical” copper red recipe that used zinc oxide. He discussed the alkalines, magnesium and calcium fluxes, and their effects on the glaze, but gave no reason why the zinc was included.
The Studio Potter magazine “copper red issue” (Vol. 8, No.1, 1979) also presented recipes that included zinc and barium, but with no indication as to why. One exception was the George Wettlaufer recipe; he stated that “an addition of 5% zinc oxide seems to help deepen the color.”
“Research on Copper Reds and Celadons” by C. Lauth and G. Dutailly, published in 1888, was the first modern scientific study of copper red glazes. Robert Tichane, quoting from this study in his book Reds, Reds, Copper Reds, says that Lauth and Dutailly found that “zinc associated with barium gives the most beautiful reds.” Perhaps this is the reason that, 100 years later, potters are still using zinc and barium in their copper red recipes.
Tichane speculated that, because sulfur gives a bad livery color to copper red glazes, perhaps Lauth and Dutailly used zinc and barium to chemically tie up the sulfur. He also states that, although a 1% or less addition of zinc will not change the color enough to notice, it is undesirable because of its effect on color and its volatility.
My motivation for testing what barium carbonate and zinc oxide contribute to copper red glazes was both economic and safety related. Zinc oxide is one of the more expensive fluxes used in ceramics. Some authors, such as Ian Currie in Stoneware Glazes, a Systematic Approach, and Frank Hamer in The Potter’s Dictionary of Materials and Techniques, contend that zinc oxide is a worthless addition to any reduction glaze because it is volatilized at a low temperature and thus adds nothing to the final glaze. Barium carbonate, on the other hand, is dangerous to the potter in its raw state and to the final user of the ceramic product if the glaze leaches. Obviously, we potters would be better off if we could achieve the same glaze results without using zinc oxide and barium carbonate.
Strontium carbonate has recently been suggested as a nontoxic substitute for barium carbonate, so it was used in two of the five variations tested (see Recipes chart). All five were tested over several firings. Glaze 1 included barium carbonate and zinc oxide. In Glaze 2, strontium carbonate replaced the barium carbonate. Glaze 3 replaced barium carbonate with strontium carbonate, and omitted zinc oxide. Glaze 4 included barium carbonate, but omitted zinc oxide. Glaze 5 omitted both barium carbonate and zinc oxide.
In the variations where barium carbonate and zinc oxide were eliminated, whiting (calcium carbonate) was increased to replace the lost fluxes. In the variations where barium carbonate was replaced with strontium carbonate, a ratio of 3/4 strontium to 1 barium was used, with calcium carbonate replacing the extra 1/4 in the ratio.
Tin oxide and copper carbonate, treated as additives, were not included in the empirical formulas (see chart). Given that some authors maintain that zinc oxide is vaporized in a Cone 10 reduction firing, the recipes that include zinc oxide were calculated to both include and exclude it as a flux.
One-thousand-gram test batches of each of the five glaze variations were made at the same time, using the same materials. Seven bisque-fired test tiles were glazed with each glaze variation. The tiles were dipped in the glaze, then the top halves were dipped again for a thicker coating.
Over the next several months, a set of the five variations was included in each firing of my kiln, a 40-cubic-foot wood-fired downdraft. The five tiles in each firing were grouped in a tight circle with their bases touching. I attempted to achieve uniformity between firings, but firing a wood kiln, even with years of experience, is necessarily an imprecise science.
My basic firing schedule was approximately 3 hours of oxidation to Cone 010, followed by 1 hour of heavy reduction to Cone 1, and 3 to 4 hours of light to medium reduction to Cone 10. At a “flat” Cone 10, the kiln was sealed shut at the fireboxes and chimney, and allowed to cool slowly for approximately 48 hours.
The purpose of this exercise was not to measure objective results, such as glaze fit and durability; the necessarily subjective criteria were color, surface quality and, most subjective of all, beauty. After spending literally hours studying the test tiles, I could not discern any positive variations in the glazes that resulted from the different recipes. Usually, in the world of scientific methodology, a result of no positive differences among the variables is bad news, because the point of an experiment is to identify and document cause and effect. The results from my tests indicate that the addition of small amounts of zinc oxide and/or barium carbonate to a copper red glaze makes no difference in the appearance of the fired glaze.
As is usually the case with copper red glazes, there were plenty of variations between the test tiles. There were definite variations between the test tiles from different firings, but the variations were consistent among the different glaze variations in the same firing. This indicates that the firing is much more important than minor variations in the glaze composition.
There were also differences that can only be described as random; in one firing, Glaze 1 may have looked slightly better than the others, but in the next firing, Glaze 3 would look better, and so on, with no discernible pattern.
For close to a year since the conclusion of my glaze variations tests, I have been using Glaze 5 (no barium carbonate, no strontium carbonate, and no zinc oxide) on my production pottery. With a nod to that 1980s band, I have named it Simply Red. Comparing these pots to those produced during the several years I used Glaze 1 (with barium carbonate and zinc oxide), I can discern no difference since the switch. Another benefit of Glaze 5 is that it has a slightly lower expansion than the other variations. Copper red glazes are prone to crazing because they generally have low levels of alumina and silica, so the higher levels of alumina and silica in Glaze 5 make it less susceptible to crazing.
All this adds up to good news for potters who want to remove expensive and hazardous materials from their copper red glazes.