WHITE GLAZE

Two versions of a distinctive opaque white glaze (as opposed to a translucent or transparent glaze appearing “white” over a white slip or a pale clay body) occur in Mainland Southeast Asia. Although differing in composition, the two variants were used on vessels of similar form and decoration in roughly the same time period. They may be different technological responses to a common source of inspiration—Chinese cobalt-decorated porcelain.

Opaque white glaze was used on earthenware speculated to have been made in Lower Burma (Hein 1995). This ware first came to attention in the 1980s, when examples were found in grave sites excavated along the Thai-Burmese border (Shaw 1986, 1987, Brown 1988; Machida City Museum 1996); no examples are included in the Freer and Sackler collections. The white glaze is composed of lead and silica and is opacified with tin (between 25 and 37 percent lead monoxide; between .18 and .58 percent tin dioxide) (Emoto and Hasebe 1986, Yamazaki et al. 1989, Yamazaki and Murozumi 1990/91–93, Yamazaki 1996). It has been suggested that this use of opacified lead glaze was introduced to Burma from West Asia (Di Crocco and Schulz 1985). Another variant of lead-glazed earthenware was made in Arakan in the sixteenth and seventeenth centuries (Hein 1995, Tsuda 1999). The relationship between the two products is not yet understood.

Applied over vessels made from a red earthenware body—dishes with wide unglazed bases, bowls, footed bowls and dishes, and small jars—the glaze seems to have been intended to replicate the appearance of porcelain. Some vessels are monochrome white, green (tinted with copper), or reddish-brown (tinted with iron), while others are adorned with “inglaze decoration” applied by trailing copper-green-tinted glaze over the white glaze before firing. The vessel shapes and the format of the decoration suggest a relationship to Chinese cobalt-decorated porcelain of the late Ming dynasty, and thermoluminescence (TL) dates give a range from the second half of the sixteenth century to the early eighteenth century (Yamazaki 1996). Vessel shapes and designs also bear a resemblance to the Sawankhalok wares of the Late Stoneware (LASW) phase, associated with the fifteenth and sixteenth centuries.

A matte, opaque white glaze was applied to Sawankhalok stoneware made at the Pa Yang kiln group. The high-temperature glaze was formulated with low alumina (58.5 percent), high magnesium (3.6 percent), high calcium (21.6 percent), and high phosphate (3.3 percent) (Hein 2001, 296, averages of eleven samples). According to Roxanna Brown (1993, 46), “the secret to the white glaze of Sawankhalok was the addition of what in Thai is literally called ‘horse-tooth stone.’ What exactly this ingredient was, it did contain a high calcium content.”

The white glaze was used alone on footed bowls, alms bowls, narrow bottles with loop handles, and small, round jars with four lugs on the shoulder and a domed lid. It was also used in combination with a golden-brown glaze in the format sometimes called “pearl and brown”; both glazes were applied with a brush over finely detailed incised decoration, most typically on covered boxes. Evidence from known shipwrecks suggests that opaque white glaze began to be used at the beginning of the sixteenth century and continued through the early seventeenth century, when export tapered off (Brown 2004). A precedent for the high-temperature opaque white glaze used on Sawankhalok ware may possibly be found in the “moon white” glaze used on stoneware made at the Shiwan (Shekwan) kilns in Guangdong province. It might be reasonable to propose transmission of this stoneware glaze via Chinese merchants trading in Sawankhalok ceramics in Ayutthaya.


field notes

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Thursday, April 2, 2009 | 12:59:00 PM | posted by: LouisKatz

I have always translated hin fan maa or "horse tooth stone" as feldspar. I have read that feldspar, when freshly cleaved, has a horse-tooth-like luster. Another translation I found on the web lists แร่หินภูเขาไฟ, literally "volcano ore stone". I believe this is a more general term for feldspar, but I am not sure.

The high P2O5 content leads me only in a few directions in terms of mixing this glaze. One source for P2O5 is wood ash. A low-iron wood ash would be needed and it would probably need washing, or need to be gotten out of a firebox where most of the alkali metals (Na, K) had volatilized out of the ash.

The other seeming likely source is bone ash. The glaze formula can be approximated by 2 parts wood ash (washed) and 1 part each clay and silica. More accurately 56% wood ash, 20% EPK kaolin, and 24% silica. It can also be mixed using bone ash 6%, limestone 30% (with some magnesia content), EPK kaolin 28%, and silica 34%. My suspicion is that both the shells and the reference to feldspar are bad leads.

After reviewing Don Hein's glaze composition from 11 samples in the region [see below], my belief is that these glazes were made with wood ash, clay and silica (perhaps rice-hull ash). The average recipe above can be made at least approximately with a combination of washed wood ash, unwashed wood ash, clay and silica. Since wood ash is never a constant, the wood changes with region and type of wood, and the amount of alkali metals (soda and potassia) changes with the temperature the ash was burnt at or how or if it was washed-it is really hard to come up with a firm recipe. It also seems likely that the potters were aware of different effects of different woods and consequently the differences in the ashes. (It is unlikely that the type of fuel stayed constant over the long years of production.)

When wood ash is obtained from a firebox it is much more like washed wood ash, as much of the alkali metals have been volatilized in the firing. (This volatilized alkali gives pots away from the firebox and protected from falling ash a "soda" glaze.) Washing similarly removes the lye or sodium hydroxide from the ash.

The average of 11 samples can also be approximated using a  mixture of washed and unwashed wood ash 46%, kaolin 12%, and silica 40%. This is less kaolin than my first effort but still points to a composition easy to make with available ingredients. There is also the possibility that some of the material was rice-straw ash. To my thinking the choice of ash was probably critical for this glaze.

In the percentage glaze analysis I find no reason to believe that hin fan maa (feldspar) was used or needed. The phosphorous could be had from ash. There is not enough silica (or alumina) in this glaze to account for use of a feldspar-whether soda or calcium spar-unless there is no clay in the recipe. This seems odd. In addition, using a calcium spar would rule out the source for the phosphorous (the ash).

[Editor's note:
Average glaze composition of 11 white glazed samples:

SiO2 58.5%
TiO2 0.2%
Al2O3 9.3%
Fe2O3 0.7%
MnO 0.1%
MgO 3.6%
CaO 21.6%
Na2O  0.7%
K2O 2.0%
P2O3 3.3%
From Hein 2001, 296.]

Louis Katz – Professor of Art
Texas A&M – Corpus USA