Glazing entails application of a substance called glaze on the surface of the object created by pottery and it is applied through various techniques like dusting, spraying, brushing and dipping. As a rule, glaze is applied before the object is subjected to fire treatment and it is diluted with water so as to facilitate ease of spreading over the surface.
One of the reasons as to why glazing is conducted prior to heating is because its color is subject to change on being exposed to heat. Therefore, it is but natural for the glazed object to appear different in terms of color before and after having been treated with fire and this is something which the craftsman needs to bear in mind. That said, the glazed object is prone to stick to the sides of the kiln and to prevent this, either the object is fired in parts or it is supported by spurs which are discarded after their purpose is served.
There are several techniques which could be employed for glazing and these are discussed as follows -
Used during the 17th and 18th centuries to construct sewer pipes, salt glazing entailed introduction of common salt into the kiln while firing was in progress so that the salt would attain a volatile state and settle on the surface of the object, thus forming a glaze. There was a time when this method was widely used especially while creating objects for domestic use but gradually lost its acceptance owing to environmental concerns. It has long since been obsolete and is hardly used nowadays.
In this method, a lot depends on the source of ash used for glazing and while the traditional source is organic plant matter, combustion waste from kilns is also a viable alternative. This method is hardly used by craftsmen, a rare example being that of pottery hailing from Catawba Valley in USA.
Typical of pottery objects which are blue and white in color, underglaze i sapplied by brush or by being poured into a mould covering the iside of the object by being swirled around.
Glazing of Porcelain
In-glaze, on-glaze and enamel are methods used for glazing porcelain objects and they differ in terms of sequence of glazing vis-a-vis applying decoration.
Irrespective of the method used, the porosity of the pottery objects subsequent to glazing is significantly reduced and henceforth these are less permeable to liquids.
The fact that fire test truly brings out a person's character is particularly applicable to pottery because it is only after being put through the kiln that a pottery object reveals its true nature in terms of shape, appearance and durability. On being subjected to high temperatures, an object of clay undergoes several changes not just externally but intrinsically too so that on emerging it is relatively harder and fused than its earlier self.
Kiln is the traditional as also the universally accepted method for firing clay and kilns across the globe are similar to each other as far as construction is concerned. Once lit, the temperature within the kiln remains constant throughout and this is an important aspect as far as making of pottery is concerned. Being cognizant of this fact often works to the advantage of the craftsman because he can then regulate the temperature as per the variety of clay and enable the object to mature well.
Heat within the kiln could be generated from several sources and while traditional options are those of wood and coal, contemporary kilns are generated by gas or electricity. It was the problem of excessive smoke and soot produced in wood and coal kilns that prompted the quest for other methods that would be clean not just for the potter but also for the environment and this is how gas and electricity were incorporated.
At this juncture, another important point that needs to be borne in mind pertains to the degree of heat required by different varieties of clay. To this effect earthenware occupies the lowest rung of the heat ladder with stoneware poised at the immediate next level and porcelain requiring the highest temperature. Given the long-lasting effect of heat, an expert craftsman can utilize this knowledge to his advantage and produce wares which are exemplary in terms of durability, functionality and design.
Within the kiln, the craftsman has a choice of creating an atmosphere that is best suited to his style of pottery. Therefore, he would create an oxidizing atmosphere if he wishes his clay to get oxidized, meaning enriched with oxygen, and a reducing atmosphere for it to be stripped of oxygen. This is particularly evident in case of clay which is rich in iron wherein it is likely to turn a rusty red on being oxidized and acquire a greenish tinge on being reduced.
Firing is an aspect of pottery that brings out the acumen in the maker and best of traits in his creations and hence needs to be mastered by every craftsman involved in this subject.