TECHNIQUES AND PROCESSES - GENERAL:
Glazing is, in my judgment, the most complicated process in ceramics. It requires greater study, analysis and observation. It is important for us to be very clean and organized in our work, and take notes of all the steps we follow to learn from our own experiences: If we want to obtain that result we liked again, we will know how to do it. From our experimentation will come our best formulas, and with them our true contributions to the art of ceramics. With our own results, we will have gotten a true personal seal that will make our work stand out from another artist’s that uses the same materials and the same techniques.
Perhaps the biggest difficulty of glazing is that it takes a great deal of imagination since the true hues only appear after firing. The glaze is a thin coating of glass applied, in the form of powder diluted in water, over the surface of a piece. When it is fired, the raw materials in it react among them, melt down and coat the piece, making its color unchangeable even if it is placed outdoors, exposed to all kinds of weather.
The glaze will be the finishing touch of the artwork and it will be so determining that from this point onward we will know if the piece will continue its life or go to the trashcan for its destruction.
Components and Temperature of Glazes
Glazes can be transparent, opaque, bright, semi-matte and matte. They are made of three basic ingredients: The first one is silica, that will form the glass; the second one, a flux to control the melting point of the glaze, which can be leaden or alkaline for low temperature –if it contains lead or sodium – or potassium or feldspar for high temperature. Alumina is the third ingredient that provides stability and enables its bonding to the clay body.
Glazes are also classified according to their maturing temperature: The ones with low temperature mature between 900 C° and 980 C°; the medium ones, in the range of 1020 C° to 1060 C°, and the high
Stacking and Firing in the Kiln
Preparing a batch of glazed pieces requires better care than one of raw pieces. Before loading a glazed piece into the kiln we should verify the glaze is totally dry, since humidity can cause defects. Also, the base of all pieces should be cleaned of glaze remains. When placing them, we will do it on refractory supports to avoid stains or drippings of glaze that would damage the refractory shelves of the kiln. To protect those and to prevent the pieces from adhering, we apply a coat of kiln wash to the shelves (alumina with kaolin dissolved in water).
During the stacking process, we should also leave space between pieces and place them about 3 cm from the walls of the kiln; for they will get fused to any surface they make contact with during the firing.
The glaze firing should be slow, although it is often faster than the bisque firing. It will begin slowly, with the air vents opened so the vapors can exit. We maintain this low rate up to 300 C°, at the rate of 100 C° per hour. From then on, we increase the heat until reaching the fusion or maturing temperature of the glaze. This firing can last from 6 to 8 hours depending on the size of the batch in the kiln and of the complexity of the pieces.
However, when the glaze reaches maturity, we do not turn off the kiln but rather keep the final temperature during 30 to 45 min, so the glaze fuse perfectly to the ceramic body avoiding pinholing
The Color in the Glaze
The coloration of the glaze takes place when oxides are mixed in (copper, cobalt, manganese, iron and nickel, among others) or when pigments are mixed in, which are industrially manufactured colorants.
The metallic oxides produce some very special colorations in the glaze, not comparable to the ones that can be obtained using pigments, which are often more flat and stable. When oxides are used, the result is a color with a lot of shades. Little particles of different metallic and black tones get fixed inside the glaze. The use of oxide as colorant yields a piece with a finishing of greater beauty, attraction and artistic level.
Even though the beauty of the oxides as colorants is indisputable, so is their complexity, therefore it is important to have tried them before applying them to the glaze, because some of their components can have an adverse reaction with it and raise or lower the ma
Pigments are easier to use, because unlike oxides, they give an idea of the future color the artwork will acquire. Oxides are grayish during their application, no matter the final color. Pigments are more stable since the manufacturer indicates the temperature they should be fired, they have a wide range of hues and they can often be mixed among themselves. Perhaps the main inconvenience is that they generate flat colors, but if we experiment on them, we will be able to obtain interesting nuances.
The glazing in ceramic requires so much study and time, it is difficult for a ceramist not to guard jealously the tests of glazes he works with, as well as his notes on the procedures.
There are four traditional ways to apply glaze: dipping, pouring, spraying and paint brushing. One is more advisable than the others according to the type work. For instance, in vases, two commonly used methods are dipping and pouring. For sculpture, the paintbrush is the most recommended in general.
In my work, I mostly use the paintbrush and the hake brush to glaze. Sometimes I use the spraying method with an air compressor, in order to achieve a finishing effect.
As to the coloration, generally I start by applying with a flat brush two or three layers of white-matte glaze, on which I paint successive layers of bright glazes of many colors, obtained from pigments and oxides – previously tested –, in order to get a lot of hues in a same piece.
When using a matte glaze as base, the final layers have a very controlled brightness that I need to achieve harmony between the