Unit 7 Treatment after Firing: Grinding and Glazing
Many ceramic products can be taken directly from the kiln, inspected, and shipped to the customer. Certain other products, however, require additional processing to meet customer specifications. These postfiring processes are grouped under the general category of finishing operations and may include grinding to meet critical size or surface finish requirements and application of adherent coatings for protection, decoration, or other special needs. Technical ceramics are carefully examined for flaws before shipment to the consummer. Cracks and pits can sometimes be detected visually or by dye penetration tests. X-ray and ultrasonic techniques may also be used to detect flaws.
Grinding
Most ceramic products can be fired to meet customer specifications on dimentions without further processing. By calculating the proper oversized dimensions of the formed part from dimensions specified by the customer, and by knowing the shrinkage of the ceramic associated with drying and with firing, a manufacture can usually fire to a given size and shape specification. Frequently, special firing techniques are required to insure meeting a specification. The ceramic parts may have to be supported by refractory kiln furniture to prevent warping or slumping during firing. Solid refractory setters or setters of the same composition as the parts being fired may be placed under certain parts to help control shrinkage and warpage. A layer of refractory grain such as calcined clay or fused alumina may be required to keep parts from reacting with or sticking to setters or refractories during firing. Tubes and rods are frequently hung by collars into refractory saggers so that gravity tends to keep them straight during firing.
If the manufacture cannnot meet the dimensioinal and surface finish specifications of the customer by special firing techniqures, then the ceramics must be ground and (or) polished after firing. Diamond tooling is usually rerquired to grind hard materials such as alumina ceramics, but silicon carbide, alumina, or other abrasives may be used for softer materials. Disks may be lapped to the proper flatness, thickness, and surface finish. Centerless grinders are often used to grind the outer diameters of cylinders and rods. Small parts as well as the diameters of disks or the ends of cylinders may be ground on tool post grinders.
Very close size tolerances are commonly required on technical ceramic parts. The dimensions of these parts are frequently measured under carefully conotrolled humidity and temperature conditions with specialized measuring devices. Special grinding and polishing techniques are always required under these circumstances.
Glazing
Many ceramics are glazed after firing. A glaze is a special glass coating designed to be melted onto the surface of a ceramic body and to adhere to that surface during cooling. Glazes are used primarily to seal the surface of a porous ceramic to prevent absorption of water or other substances. The resulting smooth impermeable surface is also attractive and easy to clean. In high tension insulators, a glaze ensures maintenance of good electrical properties even in the rain. Special colors and textures can be developed within the glaze to provide decoration and sales appeal.
The major constituent of a glaze is generally finely ground silica, with the addition of constituents rich in alkalies such as sodium and potassium oxides to lower the melting point of the glaze and alkaline earths such as calcium oxide to impart chemical durability. Lead oxide and boric oxide are also frequent glaze constituents. The overall composition is always adjusted in order to control the thermal expansion of the glaze, which must be equal to or slightly less than that of the underlying body. Additives are used to color the glaze or make it opaque.
Most glazes are prepared by wet-grinding together various raw materials along with a specially prepared commercial frit. The frit is a glass containing all originally soluble materials, all coloring oxide, and those materials which are toxic in uncombined form. Frits are produced commercially by melting and quenching a glass made up of the required chemical constituents. Quenching may be accomplished by pouring the molten frit directly into water or by roll quenching in which the frit is quenched between water cooled steel rolls. After quenching, the frit is ground, dried, bagged, and shipped to customers for use in formulating glazes or enamels.
To make the glaze, the frit is placed in a ball mill along with clays and other insoluble materials and milled to a definite particle size with the proper amount of water. Binders may be added to aid in application. The milled glaze slip may be applied to the ceramic by spraying or dipping, and patterns can be added by printing. Spraying is used in many automated processes. The powdered glaze dries rapidly on the ceramic surface, and special drying is usually not required before firing.
Glazes can be applied to green ceramics and to completely vitrified ceramics; however, ware is generally bisquefired before glazing. Bisque firing is a low temperature firing that removes the volatiles from the ware and accomplishes part or all of the firing shrinkage, thus assuring better success in the glazing operation. The body and glaze are then finish fired together (glost fired). Electrical ceramic ware is frequently completely matured and then glazed at a lower temperature than the maturing temperature of the body. Low-cost items such as pottery can frequently be sprayed in the green state and the body and glaze can be matured together. In the china process the ceramic body is first fired to maturity; then the glaze is applied and matured by firing at a lower temperature. In the porcelain process, the bisquefired body and glaze are matured together in the final glostfiring opoerations. It is usually necessary to warm previously vitrified ware before the glaze slip is applied or it will not adhere properly.
Various glaze defects can occur in the different stages of production. Crawling (uneven coverage) occurs when the glaze slip does not satisfactorily wet the body. This can often be corrected by changing or increasing the amount of organic binder in the glaze slip. Crazing (fine network of cracks) occurs if a matured glaze has a higher coefficient of thermal expansion than the body. Shivering (shearing asway of the coating in spots) occurs when the matured glaze has too low a coefficient of thermal expansion compared to the body. Pitting can be traced to volatiles in the glaze or body.
Selected from "Ceramics: industrial processing and testing", J.T.Jones and M.F.Berard, Iowa State University Press, Ames, Iowa, 1993
Words and Expressions
kiln 窑，炉
flaw 裂纹
frit 熔块
quenching 淬火
collar 套管
sagger 匣体
abrasive 磨损的
glaze 上釉
impermeable 不透水的
alkali 碱性
vitrify 玻璃化
bisque firing 素烧
glostfiring 釉烧
crawling 缩釉
crazing 龟裂
shivering 脱釉
shearing 剪切
