Publication: Journal of the American Ceramic Society
Columbus, OH, United States
DUPLICATION OF CHROME-BROWN ELECTRICAL PORCELAIN GLAZES WITH
BY RICHARD B. FULLER AND ARTHUR S. WATTS
This condensed report covers the development of a commercial glaze using cheap, nonessential materials in maximum amount. The results meet fully the more expensive glaze which is no longer available.
As a result of wartime restrictions in. the use of chromium salts, a study of the duplication of the brown glazes used on most electrical porcelain insulators was undertaken with the object of utilizing only those materials which do not involve priorities.
Before the introduction of these chrome-brown glazes, most porcelain insulator glazes were made from mixtures of slip clays with small additions of feldspar and flint. These glazes were semitransparent, reddish brown in color, and very sensitive to kiln atmosphere. A later development was a mixture of slip clays with a manganese dioxide content ranging up to 6%. This glaze was a rich chestnut brown with a brilliant surface, but it had a tendency to show crystalline mat spots where the application was thin.
The clays used were chiefly Albany and Michigan slip clays, which are being employed extensively on stoneware today. They produce a brownish black glaze at cone 7 when an excess amount of the Albany slip clay is used and a chestnut brown glaze when more Michigan slip clay is used. All these glazes develop a rust-brown surface on edges and where they are thin when the heat-treatment exceeds cone 7.
Starting with Albany slip as a base, a series was prepared using Florida kaolin, BaC03, Mn02, Fe203, and ZnO in amounts up to 10%. When this glaze was applied to an electrical porcelain body and fired to cone 106, the samples showed fading in color on the edges and wherever their application was thin.
Blends of these glazes were made and fired, but nothing of promise developed except in those containing more than 6% of MnO2 and less than 4% of ZnO, which were a dark red-brown with good covering qualities. None of these, however, was similar to the glazes to be duplicated, and all were slightly overfired at cone 10° except when at least 8% of Florida kaolin was added.
A base glaze was therefore prepared, consisting of 92% of Albany slip clay plus 8% of Florida kaolin, and a test series was made by adding 2% and 4%, respectively, of BaCO3, MnO2, and TAM ceramic stain. This stain contains materials which do not involve priorities. It is an extremely fine powder with a composition of ZrO2 5.0%, SiO214.5%, TiO2 11.75%, A12O, 28.15%, Fe2O3 24.2%, Cr2O3 10.15%, Na2O 0.9%, and CaO 0.5%.
Glazes containing 2% and 4% of BaCO3 are a neutral brown rather than a red-brown; those with 2% and 4% of MnO2 retain their red-brown color but are semitransparent; and those containing 2% and 4% TAM stain are very opaque and of a deep chocolate- brown color with a good brilliant surface. Blends of the glazes containing 4% of MnO2 and 4% of TAM stain are the most promising, especially when the TAM stain glaze predominates.
The introduction of small amounts of Michigan slip clay to replace Albany slip clay was next studied. A blend containing up to 8% of Michigan slip clay with 80% of Albany slip clay, 8% of Florida kaolin, 1% of MnO2, and 3% of TAM stain is definitely promising, but specific gravity and thickness of glaze application must be carefully controlled if color uniformity is required.
To develop a glaze which in spraying or dipping is similar to the present electrical porcelain glaze, a typical white, electrical porcelain glaze was prepared to which 15% of TAM stain and 3% of MnO2 were added. The clay content was also replaced with Albany slip clay. A mixture was then prepared of two-thirds of Michigan slip clay plus one-third of Albany slip clay. These glazes were blended in various proportions, applied to electrical porcelain trials, and fired at cone 10-6 in a standard electrical porcelain tunnel kiln. The most satisfactory product has the following composition:
Potash feldspar 33.75%
Mich, slip clay 16.75%
Albany slip clay 8.25%