| Introductory Science Concepts | Hands-On Activities |
Overview
Students apply glaze to the previously bisque fired pieces they created
during the Clay I session. The bisque firing, at a temperature of about
1800oF, removed water from the clay, making it hard while maintaining a
porous surface that absorbs glaze. The glazed pieces are loaded into a
gas fired kiln and the kiln quickly heated. While the kiln brings the clay
pieces to a high temperature, the instructor informs students about the
science and art of glazing. Students then remove their pieces, still glowing
hot, from the kiln and place them in dry pine needles. This firing and
reduction process is known as raku.
A glaze is, in fact, a glass. Glazes are usually applied to low temperature fired clay ware(bisque). The fired glassy coating provided by a glaze serves to seal the clay body, making it impermeable to gases and liquids. The technique of glazing has evolved to the point that the color, texture, and opacity of a glaze can be infinitely varied to produce highly decorative and artistic effects.
A glaze begins as a suspension of finely ground minerals in water. The various powdered minerals are carefully weighed, or measured by volume, then mixed with just the right amount of water to produce a thick, homogeneous suspension which has the consistency of heavy cream.
Fundamentally, a glaze must include a glass forming mineral, usually silica, a flux to lower the melting temperature of the glass former, and a modifier to increase the viscosity of the glass and enable it to adhere to the clay body during the firing process. Certain transition metal oxides may be added for color and the oxides of titanium, zirconium, or tin may be used to create a translucent or opaque effect.
[Chemistry
2.01]
[Physical
Science 5.06]
The following diagram illustrates the categories of minerals according to their function in the glaze. Formulation of a glaze would be a relatively simple task if each compound functioned in only one way.
One complication in this scheme is that each compound, other than silica (SiO2), may perform more than one function in a glaze. For example, boric oxide, B2O3, is categorized as a modifier, but it is also a flux and a glass former.
Another complication arises from the source of the minerals. In the diagram above, all of the minerals are indicated as oxides. Unfortunately, most minerals do not naturally occur in the earth as pure oxides. Quite frequently, they occur as mixed silicates, carbonates and oxides. It would be a daunting and prohibitively expensive task to synthesize and purify each of the oxides shown in the table. A knowledge of the percent composition of naturally occurring minerals is necessary in order to determine the mass of each element present. An even more useful calculation converts the mass of each element to chemical equivalents or moles. In this way, the contribution of an element occurring in more than one mineral may be summed and adjusted to provide the optimum amount of that element for a particular glaze formula.
The physical properties of mixtures play an important role in the formulation of glazes. The melting or softening temperature of a mixture is lower than the melting point of the highest melting component. In some cases, called eutectic mixtures, the melting temperature of the mixture is even below the melting point of the lowest melting component. Since a glaze must melt within the firing temperature for a particular clay body, spread over the clay body and seal the pores, but remain viscous and adhere to the vertical surfaces of the object, it is critical to know the liquid range and viscosity of a given formulation. Most often, experimental test tiles, coated with a glaze formulation, are fired in a kiln to determine whether the glaze will be suitable under the conditions to be used for the desired product.
Another factor to be considered is the coefficient of expansion of the glaze. If a uniform, consistent finish is desired, the glaze and clay body must shrink to similar extents during firing. If the glaze shrinks more or less than the clay, the surface of the glaze will crack and craze. This is actually a desired effect in some instances, but it should be anticipated.
[Chemistry
1.04]
[Physical
Science 5.04]
Only a few of the variable factors which impact the appearance and properties of glazed ceramics have been considered here. There are many more. Centuries of acquired knowledge about the properties and behavior of clays and glazes have produced an archive of information available to modern ceramists. This core of information provides a foundation on which an individual artist or craftsperson can build to produce a combination of techniques and formulas that characterize his or her work.
The technique of raku firing involves heating clay objects to around 2000oF. At this temperature, the clay body itself may undergo some vitrification and the applied glaze melts and spreads to cover the surface of the clay. The kiln is opened while the clay pieces are still glowing hot. The pieces are removed from the kiln with long handled, metal tongs and plunged into a bed of dried leaves, straw, or other flammable material. Combustion of the flammable material removes oxygen from the atmosphere around the clay pieces and, in some cases, from the oxides in the glaze. This process is known as reduction. It can be thought of as a competition for oxide ions between the carbon and hydrogen atoms of the combusting material and the oxide in the glaze. The winner forms the strongest bonds to oxygen under the prevailing conditions.
[Chemistry
2.05]
[Physical
Science 6.01]
If a glaze contains metal ions that can exist in more than one oxidation state, the metal may accept electrons from the combusting material and allow the now excess oxide ions to combine with carbon and hydrogen atoms. Different oxidation states of transition metal ions have different colors. Thus, the colorant substance in a glaze may change during this process to exhibit hues of all its possible ions and, in some cases, the color and shine of the metal itself. On occasion, carbon atoms from the combusting material, which become incorporated into the glaze, provide areas of blackened surface. The results of this rather chaotic combustion and reduction event are quite unpredictable and provide a wealth of surprises to the artist or craftsperson.
[Chemistry
2.03]
[Physical
Science 6.02]
Two buckets containing suspensions of glaze forming minerals are provided for the students' use. One of the glazes, called Del Francio Lustre, contains the following:
78% Gerstley borate - a hydrated mixture of calcium and boric oxidesThe Basic White Crackle glaze is formulated to shrink differently from the clay body and produce a crazed surface of tiny cracks.
20% Cornwall stone - a mineral containing aluminum and silicon oxides and a
variety of alkali metal and alkaline earth oxides
2% copper(II) carbonate - CuCO3
The copper(II) carbonate will decompose to copper(II) oxide and carbon dioxide gas at the temperature of the kiln. During the reduction process, some or all of the blue-green copper(II) ions may be reduced to red copper(I) oxide and/or reddish gold copper metal.
The other glaze, called Basic White Crackle contains:
65% Gerstley borate
5% Old Mine #4 ball clay - a mixture of aluminum and silicon oxides
15% Nepheline syenite - a mixture of aluminum and silicon oxides along with
sodium and potassium oxides
10% Tin(II) oxide - SnO
5% Flint - SiO2
The students thoroughly mix the suspension to produce a homogeneous, creamy liquid. Then each student applies one or both glazes to his or her creations by dipping the object in the glaze or brushing the glaze onto the surface of the object in selected areas. Students attempt various effects. They may dip half a piece in one glaze and half in the other glaze, perhaps overlapping the glazes in some places. They may brush the second glaze onto a piece that has already been immersed in the first glaze. Or they may brush a glaze on portions of the clay and leave the rest of the surface unglazed. Unglazed surfaces often become black after firing and reduction due to carbon particles trapped in the pores of the clay. Students are encouraged to employ as many techniques as they wish. They must, however, remove glaze from the surface of the object that will touch the shelf of the kiln to prevent the piece from sticking to the shelf.
Students then place their pieces into one of two kilns making sure that glazed surfaces do not touch each other or the shelf or sides of the kiln. The kilns are located outdoors at a safe distance from flammable materials. The kilns are enclosed by refractory bricks and have counterweighted lids. The lids are lowered during firing.
When the firing has proceeded for a sufficient time to bring the pieces
to a yellowish white glow (judged by an experienced teacher), the kiln
lid is raised. Protected by goggles and heavy gloves, students, using long-handled
tongs remove their pieces from the kiln and bury them in piles of dried
pine needles contained in metal tubs. The pine needles immediately ignite.
Combustion is controlled by placing a metal lid over the tub. Much smoke
is produced by the oxygen starved fire. After 20-30 minutes, the pieces
are removed from the ashes and allowed to finish cooling. The pieces are
then washed and scrubbed to remove loose carbon and other combustion products.
Cleaned surfaces of the ceramic pieces reveal a wide variety of hues and
textures. Each piece is different in appearance from the others, with surface
colors ranging from white to gray and pink to brown and black. Surface
textures range from glossy to matte.
