Glaze & Clay Tutorial - 5

glz3b.txt    Clay and Glaze Formulation     Robert Fromme

   Notes on Calcium, the 'Good Old Work Horse' Glaze Flux

                         CALCIUM           (Alkaline earth)
                         Oxide Formula:    CaO
                         Molecular Weight: 56

* Melting Range:

With the relatively high melting point of 2575 F.,calcium is
not an active flux at lower temperatures and is seldom used to
encourage melting below cone 4.  By adding calcium to a low
temperature boron glaze can produce mattness.  Calcium is not
an active melting flux until the middle and higher temperature
ranges.  At very high temperatures, such as cone 12 or cone 13
porcelain, calcium is a dependable glaze ingredient.

* Viscosity:  

Calcium is use to reduce viscosity in glazes which are high in
silica, but if the melt is too fluid, devitrification may take

* Melting and Mixing Characteristics:

Most glazes contain some calcium because it is a very common
and inexpensive substance and usually adds only good
characteristics to a base glaze. In the middle and high
temperature ranges, calcium is rated as the most dependable
fluxing oxide. When calcium is used in large quantities, it
can help to create matt surfaces in glazes. High molecular
amounts, .8 or more, in a lower alumina glaze gives matt
qualities through the formation of calcium silicate crystals
or lime matt glazes.  These surfaces can be very pleasant as
long as the zinc content is kept low to avoid the formation of
large disruptive crystals in the cooling surface. 

Even when the craftsperson has worked to produce a transparent
glaze mixture, calcium seldom produces a extraordinary high
gloss or 'brilliant' surface. However, in high-fired glazes
which contain calcium oxide in the range of .5 to .7 molecular
equivalents tend to become transparent and fairly bright. 
These are hard, fairly smooth, and remind one of polished jade

Functional high-fired glazes can be created from very simple
combinations of 83 to 86 parts feldspar to 17 to 14 parts
whiting.  Another simple plan suggests the addition of 60
parts feldspar, 18 to 20 parts flint and  20 to 22 parts
whiting.  Without the addition of clay or bentonite, however,
these mixtures will quickly settle out of suspension in the
glaze bucket.

*Surface Softness or Hardness:

When calcium is added to a glazes for any firing range, it has
increased durability and hardness.  This is particularly
important when it is added to low-fired glazes which have a
high quantity of lead, sodium or potassium. The Calcium
renders the mix harder and less soluble.

* Clay-Glaze fit:

Calcium tends to react with the clay body under the glaze to
create a good interface and encourage the clay-glaze fit.  It
has a middle to slightly high expansion and contraction rate
in glazes.  CaO promotes a lower expansion then the alkalies,
sodium and potassium, so it can add stability and fit in
combinations of these fluxes and silica.

In very large quantities, it may develop crazing and glaze fit

At lower temperatures, CaO and Wollastonite are used in clay
bodies as fillers which have the effect of lowering effects of
firing shrinkage and thermal shock failure.

One traditional method of gaining a good clay-glaze fit
involves the use of similar materials in both the glaze and
the clay body.  This practice is particularly applicable for
the porcelain craftsperson. Many porcelain formulae can be
changed into a glaze composition by the addition of 18% to 22%
additional feldspar and 9% to 12% additional calcium. 

* Color Response:

Calcium works with most metals for a strong and pleasant
effect on most colorants. Some clay artists have noted a
slight tendency for calcium to have a bleaching effect on iron
oxide in some glaze mixtures. 

When calcium is used with potassium and sodium, copper reds
are possible in reduction atmospheres at middle and high

Calcium has long been a choice flux along with large amounts
of barium to encourage the development of celadon green from
iron in reduction atmospheres at middle and high temperatures.
When colemanite or Gerstley borate is used a source for boron
and calcium, the glaze may develop a wonderful opalescence.

* Problems and precautions for use:

One will seldom find calcium creating a problem in a glaze and
it is usually the safest and most reliable inclusion at the
middle and high temperatures.  If a glaze has excess calcium
or if it is too low in viscosity, the glaze will easily
devitrify,  forming crystals of calcium feldspar.  A matt
glaze will result unless the alumina content is increased to

We should note that one source for calcium, bone ash is
slightly soluble in water and gives off phosphorous oxide
which may cause extra bubbling some glazes. 

Fluorspar, another source for calcium, emits fluorine during
firing and can cause extra bubbling and the loss of gloss in
some glaze surfaces.

Although Calcium Chloride is not usually thought of as a
source for calcium in a glaze, a mixture for glazing which has
had too much water added may be made thicker by the addition
of three to four teaspoons per gallon of glaze.

     Brief Notes on Calcium Glaze History 

Calcium in Historical Glazes

Calcium is a favored ingredient in most glazes throughout
history and involving a wide range of temperatures and
processes, from low-temperature lead glazes the intense
temperatures of porcelain. 

Calcium Borate Mixtures

When glazes contain formidable amounts of Gerstley borate,
colemanite or a calcium borate frit and are cooled slowly, the
slight recrystallization will yield beautiful clouds of
lighter or milky, opalescent surface over a glossy, more vivid
background melt. Some clay artists prefer the use of a calcium
borate frit for the milky opalescence because, as we have
noted, colemanite and Gerstley borate boil and bubble in the
early melt, creating a problems of mottling and sputtering in
the melt.


                    Notes on Zinc Oxide  

                         ZINC OXIDE         (metal)         
                         Oxide Formula:     ZnO    
                         Molecular Weight:  81.4

* Melting Range:

With the relatively high melting point of 1800 C.,zinc is not
an active flux at lower temperatures and is seldom used to
encourage melting below cone 01.  Zinc is not used as an
active flux, but in smaller quantities it can be used as a
replacement for lead as a flux at the lower temperatures. 
(See Bristol Glazes)  It has a fairly broad firing range from
medium to low temperatures to normal limits for higher
temperature ceramic production. At very high temperatures,
zinc will become volatile, creating some increased bubbles and

Zinc is usually introduced into glazes as an auxiliary or
secondary flux, especially in leadless glazes fluxed with
feldspars or boron. Since it is not a powerful flux at the
lower temperatures, it will act a refractory in glazes
maturing below 1050 C.(1920 F.)  At higher temperatures, it is
valuable to provide a smooth transition from sintered to a
molten state.

* Viscosity:  

Large quantities of zinc in a glaze can create problems ow
crawling due to the high viscosity of zinc. When gas bubbles
move through a high zinc glaze, the viscosity may not allow
for the surface to smooth out or heal, resulting in pitting,
pinholing and crawling.

* Melting and Mixing Characteristics:

Zinc will function as a catalyst in slip glazes and in small
amounts its contribution to glaze mixtures is usually
positive. In certain glaze mixtures it is a very powerful flux
with small to medium additions to the glaze.

Small additions seem to reduce boiling during firing and in
combination with titanium in a mixture which has very low
amounts of alumina, zinc has the tendency to help the
formation of crystals. The zinc-titanium glazes usually
require a soaking or very slow cooling temperature around 1500
F. or 817 C. and then a very rapid final cooling period.

Zinc works well to encourage a dull matt when it is used in
larger quantities at most temperatures.

Zinc oxide can produce opacity and whiteness at low
temperatures, if the calcium content of the glaze is kept low.
When used with boron, zinc can have amphoteric (neutral or
viscous) qualities at the lower temperatures.

* Surface Softness or Hardness:

Zinc can lend strength and durability to a glaze mixture.

* Clay-Glaze fit:

Zinc is in the middle of the ceramic flux list of coefficients
of expansion and contraction, so its addition in a glaze
usually works to improve crazing and related clay-glaze fit

* Color Response:

Many metal oxides will produce dramatic color responses when
zinc is used in a glaze.  Cobalt in a zinc fluxed glaze will
yield a very intense blue.  Copper greens are encouraged by
the addition of zinc in the melt.

Some metals do not respond well with a zinc fluxed glaze and
become dull and neutral.  It is a poor flux in iron brown
glazes. It works against the formation of chrome greens,
turning them brown, khaki or olive drab brown.  It is not a
good addition in copper red glazes and it may cause some high
tin white glazes to blush to a pink color.  Migration from
copper to white glazes which are high in zinc and tin have
caused some crafts people to avoid using these glazes in the
same kiln with copper red glazes.

* Problems and precautions for use:

Zinc oxide is poisonous.  Be careful with its use.

Clay crafts people prefer to use zinc oxide in its calcined
form  because water can lead to excessive drying shrinkage and
similar problems in the early stages of the firing.

Remember that large additions of zinc can create problems with
pitting, pinholing and crawling. 
Brief Notes on Zinc Glaze History 

Zinc in Historical Glazes

Bristol Glazes

The so-called Bristol glazes represent attempts to replace
lead oxide by zinc oxide when the poisonous nature of raw lead
compounds had become a health hazard in the growing pottery
industry. The use of the zinc fluxed glazes seems to have
developed near Bristol, England and were given the name from
that location.  In these glazes, zinc served as the primary
flux and calcium, magnesium, and barium were used as auxiliary

Microcrystalline glazes

These are glazes which often contain zinc, in which crystals
are so small that we are not able to detect them without
magnification.  Examples are the satin-vellum and matt glazes
formed by crystallization from solution of zinc, calcium or
barium silicates and titanates.

Macrocrystalline glazes

Zinc is also used in macrocrystalline glazes of many types. 
These are glazes in which the crystals are sufficiently large
to be readily detectable by the naked eye.  The category
includes the zinc-titanium crystal glazes and the adventurines

Satin-vellum glazes

Satin-vellum glazes often contain zinc oxide as an ingredient. 
Since true matt glazes often tend to have rough surfaces which
are easily marked by metal and which are difficult to clean,
the satin-vellum glazes provide a functional alternative.  The
much smoother semi-matt surface of the satin-vellum glazes
have a most attractive satin sheen surface and are produced by
the addition of 3 to 4% zinc, titanium and tin oxides in a
majolica or low-earthenware region (1100 C. (2110 F.).
(c) 1994  Robert Fromme          For educational use, only.

glz4a.txt   Clay and Glaze Formulation    Robert Fromme

         Notes on Magnesium as a Glaze Fluxing Oxide

                         Magnesium         (Alkaline earth)
                         Oxide Formula:     MgO2
                         Molecular Weight:  40.3

* Melting Range:

With the relatively high melting point of 2800 C., Magnesium oxide
is not an active flux at lower temperatures and is seldom used to
encourage melting below cone 3.  Adding magnesium oxide to a low
temperature glaze can produce mattness.  

* Viscosity:  

At very high temperatures, magnesium oxide encourages a more fluid
glaze, however, at lower temperatures large amounts can work with
other viscous ingredients to produce problems with pinholing and

* Melting and Mixing Characteristics:

Magnesium is used to produce beautiful opaque matte surfaces in the
higher temperature range. Reduction firing helps to produce high
temperature glazes which have the 'egg-shell' and soft 'buttery'

* Surface Softness or Hardness:

When magnesium is added to a glazes at higher firing ranges,
it increases durability and hardness.

* Clay-Glaze fit:

Magnesium tends to react with the clay body under the glaze to
create a good interface and improves glaze adhesion. 
It the lowest coefficient of expansion and contraction in glazes.

* Color Response:

Magnesium has a negative effect in the traditional celadon and
copper red reduction glazes.

It is a critical ingredient in many for the beautiful white tin
opacified glazes. 

If part of the kiln load includes copper glazes in higher reduction
temperatures, a pink blush may result in a white glaze which
contains magnesium.  The copper becomes volatile and often moves
into the surfaces of the tin and magnesium glazes near it.

When glazes contain magnesium and cobalt, one can expect the cobalt
to develop more of a purple or violet than blue. 

3 to 9% copper in a high magnesium glaze can produce a warm gray
in reduction and a cool gray in oxidation atmospheres.

* Problems and precautions for use:

Because some of the sources of magnesium are slightly soluble, it
may act as a flocculent with clay content of glazes mixtures.

Although magnesium sulfate is not a traditional raw material for
glazes, clay artist frequently use several teaspoons of it to help
thicken a bucket of glaze which has too much water in relation to
the other raw materials. 

     Brief Notes on Magnesium Glaze History

Magnesium has long been an important flux for glazes which develop
the buttery micro-crystalline matt surfaces at high temperatures.


                    Notes on Barium as a Glaze Flux   

                 BARIUM OXIDE       (alkaline earth)              
                 Oxide Formula:     BaO     
                 Molecular Weight:  153.4 

* Melting Range: 

Barium oxide does not melt until 1923 C. and serves as a useful
flux at the higher temperatures. It seems to be able to withstand
the highest ceramic temperatures where other fluxes would begin to
volatilize and boil. 

* Viscosity:  

Although barium is slow to melt and is active only at higher
temperatures, it seems to become a very active flux creating very
fluid mixtures at those temperatures. When it is used in excess it
can cause the glaze to seem more viscous due to its refractory
nature in the lower temperatures.
* Melting and Mixing Characteristics:

Ceramic artists have noted that barium seems to encourage an
additional quality of brilliance to those glazes which develop
glossy surfaces.

Barium has long been noted as an important glaze ingredient when
developing a satin matt surface.   When the mixture is over
saturated, however, the same glaze will take on a rough and
unpleasant quality of surface.  These 'stony mattes' are not only
less then pleasing to the eye, they are usually very dangerous if
used in utilitarian ware because of the toxic qualities of barium
when a glaze is permeated with it.

The eutectic between boron and barium will halt barium matt
development in a glaze and encourage fluid transparent surfaces at
higher temperatures when the two materials are included in the same

Barium has functioned as an important flux in certain unique glazes
where, at specific temperatures, the surface will develop patches
of bright gloss broken up by areas of matt and opaque glaze.

Barium oxide seems to be able to function as a dependable flux at
very high temperatures where other ingredients tend to volatilize.

* Surface Softness or Hardness:

Like calcium, barium has been included in controlled amounts to add
durability, strength and acid resistance to utilitarian glazes.

* Clay-Glaze Fit:

Barium is seldom cited as a source for problems with the fit
between clay and glaze. It does not seem to cause as much crazing
as sodium, potassium and calcium and is usually ranked near lead
and lithium in the middle of the fluxing oxides for expansion and
contraction characteristics. 

* Color Response:

Most of the metal oxides seem to produce colors which are quite
intense in glazes which contain some barium.  The color response of
cobalt and the iron and titanium in rutile in glazes with barium
are usually extremely rich. Mixtures of 1 to 5% cobalt in a barium
fluxed glaze can create unique ultramarine blue surfaces. 

Small amounts of iron oxide in a barium glaze will often produce
iron blues.  When less then 3 1/2% iron oxide is used as colorant
in reduction glazes containing calcium, barium can be added to help
produce the traditional celadon green.

In other high barium glazes, the addition of 4 to 5% iron with
around 2% stannous (tin) oxide can produce pale yellow and cream
colors. In the higher temperatures, these are frequently called
'iron yellows' and they seem to work best when they are opacified
with titanium and or zirconium.

In high temperatures oxidation atmospheres 1 to 10% Copper in a
barium glaze can create beautiful turquoise and blue colors when
the clay content is kept low. When the clay content of the base
glaze is increased, the copper will encourage the color to take on
an apple green quality.

Nickel in a crystalline, barium and zinc base glaze will yield the 
traditional 'Wedgwood blue' to surfaces which are a cool blue gray.
In oxidation, 1 to 5% nice can produce colors ranging from Prussian
blue to Indigo and violet. At very low temperatures (cone 018-011)
nickel in a glaze with barium has produced surfaces from pink to

Clay artists often mix two or three metal oxides in glazes
containing barium to produce mottled or broken colors in a surface
which may break from gloss to matt in the same glaze. 

Barium has been known to encourage mottled and streaked color
effects in some glazes.

One raw material source, barium chromate, although very toxic, is
used in low temperature glaze and enamel mixtures to achieve bright
lemon yellow, green and greenish yellow surfaces. 

* Problems and precautions for use:

Barium Carbonate is poisonous (an ingredient on rodent poison) and
many glazes containing barium are not stable even after being
fired.  Always use caution when using barium and avoid its use in
glaze surfaces which are intended for food service.  Have any
questionable glazes which contain barium tested for stability if
they are to be utilized for food or drink.

In recent years, may ceramic artists choose to replace the barium
content in their studio glazes with strontium oxide because it
gives similar properties to the glaze without the risk of toxicity
in the raw material form and in the glaze.

When drying slip mixtures and other clay bodies develop scumming
from soluble salts moving to the surface with the evaporating
water, additions of 2% barium carbonate have been used to control
this problem.  In theory, this addition converts the soluble salts
to insoluble forms through a process called 'double decomposition'.
Brief Notes on  Glaze History 

Barium has served as a secondary flux in the middle and high
temperatures.  Barium is an occasional flux in copper reduction
glaze bases in the cone 8 to 10 range. In the lower temperatures,
it has been used as a refractory and opacifier in some Raku and
Majolica mixtures.  It is an important ingredient in the Micro-
crystalline satin matt glazes for stoneware and porcelain.  As the
toxic nature of the material becomes known, we can expect to see it
used less frequently with strontium used as its replacement.


                    Notes on Strontium  

                     STRONTIUM OXIDE   (alkaline earth)
                     Oxide Formula:    SrO
                     Molecular Weight: 103.6

* Melting Range:

Strontium oxide is refractory with a melting point of 2430.  When
used in a base glaze with other materials it will function as a
flux, although it requires a slower firing or soaking in order for
its participation in the glaze melt. Its useful fluxing range spans
from cone 04 to cone 12. 

* Viscosity:  

When the temperature and composition is sufficient for strontium to
participate in the melt, it usually helps to produce surfaces which
are free from bubbles and pits.  If its content is too high, it
forces the melt to become viscous with potential for surfaces which
are not even.  Like zinc, when strontium content is too great in
the mixture, it can cause crawling.

* Melting and Mixing Characteristics:

Ceramic artists have often noted that strontium shares many of the
melting and mixing characteristic of calcium and barium.  Because
of the toxic nature of barium, strontium seems to be a reasonable
replacement in middle and high temperature glazes.

Strontium and calcium are frequently used in glazes which develop
micro-crystalline matt surfaces.

* Surface Softness or Hardness:

When strontium is used as a flux in the glaze the melt usually
results in surfaces which are hard and durable
* Clay-Glaze fit:

Strontium has a low expansion and contraction coefficient so it is
one of the best choices to avoid crazing and related problems of
glaze fit. 

* Color Response:

Calcium and Strontium seem to share many of the same
characteristics of color response.   Calcium has been noted for a
tendency to dampen or 'bleach' iron color, however, in a strontium
fluxed glaze iron will usually yield its traditional color range
with slightly more intensity.

Strontium also shares many of the color responses of barium. When
from 5 to 10% copper is used in a high barium or strontium glaze,
it can produce dark green in oxidation atmospheres. In other high
barium or strontium base glazes, 1 to 5% iron or rutile can produce
colors ranging from pale yellow and cream to yellow ocher surfaces.

* Problems and precautions for use:

Strontium is not toxic and can be used to replace the toxic flux,
barium in the middle to high temperatures.

The primary raw material source, strontium carbonate, is usually a
little more expensive than other ingredients and it is slightly
water soluble. 
Brief Notes on Glaze History

Strontium was seldom used in ancient pottery and it seems to be a
fairly recent addition to the list of fluxing oxides which are
available to the potter. Its use will increase as it serves as an
effective replacement for the toxic barium raw materials. 
(c) 1994 Robert Fromme / For educational use, only.

glz4c.txt    Clay and Glaze Formulation    Robert Fromme

 Notes on Aluminum as a Glaze Refractory or Neutral Oxide

                         Aluminum Oxide          (metal)
                         Oxide Formula:     Al2O3
                         Molecular Weight:  103

* Melting Range:

The melting point of alumina oxide is around 2050 C.. In a glaze,
however, the addition of fluxing or base oxides can cause the
refractory inclusions such as alumina and silica to enter the melt
at very low temperatures. The clay artist should keep in mind that
by controlling amounts of alumina and silica in a glaze, in
relation to the amount and type of base or fluxing oxides acts to
control the maturing temperature of the glaze. By adding alumina to
a glaze at a given maturation temperate, the refractoriness, as
well as the opacity and the viscosity of the mixture is increased.

* Viscosity:  

The viscosity of the glaze is one of the primary characteristics
in the melt which can be traced back to the amount of alumina in
the mixture.  As we have mentioned, the glaze becomes more viscous
or less runny as the alumina content in the mixture increases. In
turn, as the alumina is removed, the glaze becomes more fluid.

* Melting and Mixing Characteristics:

Alumina is both acidic and basic in its behavior. It can combine
with the acid, silica, the neutral, boron, and the bases or
fluxes.  With this behavior, like that of boron, alumina is
classified as an intermediate, neutral or amphoteric oxide in glaze
chemistry. In addition, other terms such as 'viscous agent' and
'crystal retardant' have been used to describe alumina in the

Artists and technicians who work with glass will quickly
point out that simple mixtures of fluxes and silica will produce
glass.  If those mixtures are applied to the surface of a clay
object and used as glazes they will not work well. They will melt
very suddenly and they would become so fluid that most of the
molten mixture would end up in a puddle of glass on the kiln shelf
below and under the vessel. In the cooling kiln, the glassy
nightmare would begin to crystallize or 'devitrify' into a rough,
dry, surgery surface.

Glazes are made more stable and useful to the clay artist through
the addition of alumina to the glassy melt.  With small amounts of
alumina the material is made more viscous so that it is slower to
move as it melts, so the alumina improves the chances for the melt
to remain on the clay object where it is intended to function.

With the addition of alumina, the process of change from a solid to
liquid phase is slowed and extended over a much longer temperature
range.  This has the effect of making the molten glaze last several
hundred degrees beyond the temperature that the same materials
(minus the alumina) would normally melt off the walls of the clay

In addition to improving the melt by adding viscosity and
functional range for its maturation temperatures, alumina also
serves a very important role by resisting the devitrification or
crystallization of the cooling glaze.

* Surface Softness or Hardness:

Alumina also helps glazes to become hard and durable. Since the
amount of alumina in most glazes increases as the maturation
temperature goes up, we can notice that alumina will work to
improve the hardness or resistance of glazes as their maturation
temperatures increase.

* Clay-Glaze fit:

Alumina ranks in its very low expansion and contraction with
silica and boron as one of the best ingredients to improve the
fit a glaze to the clay. The theoretical formula for clay
(Al2O3.2SiO2.2H2O) suggests that if alumina is also used in the
glaze, our chances of improving fit between the glaze and the clay
will improve.

* Color Response:

In general, most metal oxides which provide color in glazes are not
effected by the addition of alumina to a glaze. One exception to
our general assumption of color responses for alumina oxide
involves those base glazes which are intended to produce a red
glaze from copper in reduction atmospheres. Alumina content should
be kept low in these base mixtures in order to keep from hindering
the formation of metallic copper in the reduced glaze. Alumina also
has been known to darken and deaden chrome green colors in certain
base mixtures. With the addition of chrome in a high alumina glaze,
one can expect the color to turn toward brown.

* Problems and precautions for use:

The amount of alumina in a glaze is usually much less than the
amount of flux and silica. When the alumina content is too great,
the glaze will become dry and unpleasant to the touch. In extreme
cases, the glaze will simply refuse to melt. 

Feldspars and Kaolins are usually the raw materials choices for
introducing alumina into the glaze. Some situations require the
clay or kaolin to be calcined prior to use in order to avoid
cracking and shrinkage of the fresh glaze coat as it dries. Other
sources for alumina have some characteristics which one must
consider. Alumina Hydrate tends to settle in the raw glaze storage
container and it is more expensive then other sources. Amblygonite
and cryolite have volatile inclusions of fluorine and phosphorous
which can increase the boiling and bubbles which can result in
the formation of pinholes, pitting and crawling in some mixtures.


 Notes on Silicon (flint) an acid or glass forming oxide

                         SILICON OXIDE      
                         Oxide Formula:     SiO2
                         Molecular Weight:  60.1

* Melting Range:
The forms of silicon oxide, quarts and cristobalite melt at 1713 C.
while tridymite, another form of silicon melts at 1670 C. In
glazes, base or fluxing oxides are used to lower the melting range
of refractories such as alumina and silica.  The clay artist should
keep in mind that by controlling amounts of alumina and silica in
a glaze, in relation to the amount and type of base or fluxing
oxides acts to control the maturing temperature of the glaze.  By
adding silica to a glaze at a given maturation temperature, the
refractoriness of the mixture will increase.

* Viscosity:

Silicon, when mixed with a correct amount of flux, will result in
a fluid melt.  Because of the refractory nature of silicon, larger
amounts in a glaze will tend to decrease fluidity.  

* Melting and Mixing Characteristics:

Silicon oxide is the principal glass-forming ingredient in nearly
every glaze and it is indispensable for the creation of a useful 
glaze. This oxide usually makes up between 45 and 70% of the oxides
in a glaze  The glass-forming oxide combines easily with the bases
or fluxes as well as the neutrals or amphoteric oxides such as

* Surface Softness or Hardness:

Silica bestows rigidity and durability as well as tensile strength
to glass and glazes.

* Clay-Glaze fit:

The glass-forming acid has a very low coefficient of expansion and
contraction.  It works to reduce crazing and improve glaze to clay

* Problems and precautions for use:

Very small amounts of silicon oxide have no effect in glass
formation and the mixtures usually remain dry with no indication of
chemical change in the heat.  On the other hand, very large
quantities do not flux to create glass and the surface will remain
dry and unmelted.

Extensive breathing of silicon can create the disease known as
'silicosis'.  Portions of the lung begin to harden and breathing
becomes difficult. The clay artist should avoid working in
environments which are filled with silica dust and a protective 
dust mask will help to avoid the accumulation of the dust in the
(c) 1994 Robert Fromme       For educational use, only.

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