Glaze & Clay Tutorial - 7

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glz2b.txt     Clay and Glaze Formulation      Robert Fromme

Most of us have fired several types of kilns for a variety of
different processes and we have a working knowledge of the changes
which are going on in the clays and glazes as they are heated and
cooled.  We know that the glaze goes through a series of physical
changes which are similar no matter if it is a Raku firing or a
stoneware firing.


 (1.) At first the pots begin to give off steam and we can tell
that water is moving out of the walls of the clay and the glaze by

                          Kindling Temperature

(2.) If wax was used in the glazing process to protect the foot or
for wax resist decoration, that eventually reaches kindling
temperature and starts to smoke or burst into flame. Other organic
materials will also begin to burn out of the clay such as paper
used to support hand built forms, string, leaves, nuts, and other
objects used for surface decoration.  


(3.) By the time the kiln reaches a black-red heat we may begin to
see the earliest signs that a few of the lowest melting components
of the glaze start to 'sinter'.  Although one seldom sees the
sintering process, the glazes often show a few isolated, tiny,
shiny spots.  Sintering is a valuable process in the early stages
of firing.  In this step of the melt, heat converts powder into a
cohesive mass without developing a glassy phase.  What is really
happening is the corners and contact surfaces of the particles
soften in the heat and the particles begin to stick to each other
at those points.  If you have ever had to stop a firing just prior
to the fusion stage and then studied the sintered but unfused
glaze, you will understand that the temperature at which sintering
begins is well below the level of heat which is required to melt
most of the pure substances in the glazed surfaces, their 'fusion
temperatures'. Let's keep in mind that the materials in the glaze
mixtures are usually very finely ground. In the heating kiln, the
glazed surfaces have relatively large surfaces and therefore high
surface energy with relatively low melting points for most of the
components.  All of this helps to promote the sintering mechanism. 


(4.) With additional heat, some of the lower fusing materials begin
to show signs that they are trying to enter the mix as more
plentiful melting is underway.  With the rising temperature, the
lattice structures of the crystals of glaze materials begin to
loosen and new stresses are set up in the materials because of
dissimilar thermal expansions of those elements. As you would
expect, little cracks develop in the stressed crystals and particle
size is reduced in the heat. With the additional activity in the
glaze layer more particle surfaces are exposed for fusion while the
corners and edges of the crystal lattices are exposed for chemical
activity through abrasion. As the particles of crystal continue to
break up, a greater number of smaller ions and atoms are able to
enter the chemical activity, scatter, and diffuse into the melt.

In the elevated temperature, many of the glaze raw materials begin
to disintegrate into two or more substances.


As the crystals of the raw materials continue to regress the
formation of eutectics begin to develop in the liquid phase.
Eutectics are mixtures of ceramic elements which have, together, a
fusion temperate which is lower than that of any of their
individuals components. (The usual example is sodium silicate which
melts at a lower point then either silicon or sodium.)  In the
complicated melt of the glaze the more refractory particles are
eventually surrounded and then gradually taken into the molten
solution of the liquid glaze.

                     Gasses Boiling and Bubbling

(5.) As this fusion gets underway, we will see a very active or
violent period where the glazes will be boiling or bubbling.  We
have mentioned that in the heat, many of the raw glaze materials
disintegrate into two or more substances. Gasses like sulfur,
oxygen, and carbon are produced as the ceramic raw materials are
separated.   Other gasses are liberated by the bisque body under
the glazes.  Some gasses are absorbed from the kiln atmosphere, by
the liberation of gases assimilated on the surface of some of the
raw glaze particles and particularly by the unbinding of air
present in the voids between the glaze parts.  The gasses pass into
and through the fusing layer of glaze producing a productive
stirring action which helps to make the glaze more uniform or
homogeneous. The boiling of gasses in the glaze is very slow in the
early stages when the viscosity of the fusing glaze is greater, but
as the temperate elevates and as the glaze becomes more fluid the
activity is increased and vigorous.

(6.) As time and temperature continue to work on the clay and
glaze, the active dislocation of gas from the mixtures gives way to
smooth, liquefied, but viscous, glass. We have reached the maturation
temperature for the glaze. 


We need to keep in mind that the viscosity of the melt at high
temperatures is an exceedingly important attribute. (a.) Viscosity
in the glaze can be a consequence  of the temperature and (b.) the
time for which the glaze was subjected to the heat.  (c.) Viscosity
also depends upon the nature of the materials used in the
composition of the glaze.  The rate and uniformity of the fusion
process depends on those glazes which begin to fuse early and
encompass, permeate, and dissolve the more refractory raw glaze

Of course the uniformity within and the quality of thickness in a
mature glaze depends upon the viscosity of that mixture at
maturity.  Any bubbles which are not released from the glaze may
create pinholes or surfaces that look like orange-peel (dimpled)
surfaces. Because of this potential problem, most clay artists
prefer to 'soak' the kiln before it is turned off and allowed to
cool.  The additional time in the elevated temperature helps to let
the last of the gasses move on out of the liquid and the surface
tension can help it regain its uniform surface.

In drastic cases the viscous glaze may create crawling as the gas
bubbles are liberated and the surface pulls the glaze out and away
from where the bubbles moved though the molten mix exposing the raw
clay body beneath. 

We can not leave our examination of viscosity without mentioning
that the viscosity of a glaze has a repressing influence which
limits or prevents the formation of visible crystals in the cooling
glaze.  Without this constraint, nearly all glazes would devitrify
or recrystallize during the cooling.  Sequentially, transparent
glazes would be extremely difficult to create. 

                           Matt Glazes

(7.) At this point, even those glazes that will have a (true) matt
surface will appear glossy and liquid at the maturation temperature,
however, as the glaze begins to cool, some glazes will begin to
devitrify.  A thin layer of crystals, often too tiny to see, are
beginning to grow just below the surface of the matt glaze. The
coating of miniature crystals are often invisible but if they have
developed, the light will not be reflected from the surface in a
uniform or consistent manner and the object will appear to be dull
or matt. ( We should note that there are some surfaces in ceramics
which are called matt but they result from an excess of non-fusible
materials which will not dissolve in the molten glaze. There are
other surfaces which are simply dry immature or under-fired glazes
which are often called matt glazes.) 

True matt glazes, as well as other crystalline glazes such as those
opaque glazes which result from crystal growth, must be cooled slow
enough over their molten phase so that there is enough time for the
crystallization to take place. When the cooling cycle is too rapid
for these kinds of glazes, opaque glazes may remain transparent and
clear while matt glazes remain glossy.
(c) 1994 Robert Fromme           For Educational use, only.

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