2. Goals
• Better understand how glazes work
• Become familiar with glaze compositions
• Become familiar with glaze ingredients
• Learn how to mix glazes
• Learn how to compare glazes
• Understand oxidation and reduction firing
• Look at the Firehouse Studio glazes in detail
3. What is a Glaze ?
• Glass that sticks to pottery
• Glazes are mostly Silica
• Pure Silica melts at 3100F
• Add Sodium and Calcium to
lower melting point (fluxes)
• Add Aluminum to increase
the viscosity
4. Periodic Table of the Elements
Lots of elements to consider
We want to work with common inexpensive materials
Low hazard and non-volatile
5. Elements we use in the
current set of studio glazes
Of course everything is combined with Oxygen
6. Alkali Metals
Strong fluxes active at all temperatures
Lithium Carbonate (toxic)
Na Feldspar – F4 feldspar
Nepheline syenite
Sodium Carbonate – Soda Ash (soluble, toxic)
Potassium Feldspar – Custer Feldspar
7. Alkaline Earth Metals and Zinc
Strong fluxes active at higher temperature
Magnesium Calcium Carbonate – Dolomite
Magnesium Silicate – Talc
Magnesium Carbonate
Calcium Carbonate – Whiting
Strontium Carbonate
Strontium Oxide
Zinc Oxide
8. Non-metals and Aluminum
These elements go into the basic glass network
Gerstley Borate
Borax
Clays
Feldspars
Silicon dioxide – Quartz, Flint
Clays
Feldspars
Calcium Phosphate – Bone Ash
9. Transition Metals - COLOR
Chromium Oxide (toxic)
Iron Chromate (toxic)
Iron Oxide (Fe2O3 – red, Fe3O4 –black)
Copper Carbonate (toxic)
Cobalt Carbonate (toxic)
10. Opacifiers to decrease glaze
transparency
Titanium Dioxide – Rutile (>85% TiO2)
Zirconium Silicate – Zircopax, Ultrox
Tin Oxide
11. 17 Glazes from 18 Oxides derived from
27 different materials
Al2O3 Bentonite
3D BLACK Black Iron Oxide
TOMATO RED B2O3 Bone Ash
CaO Borax
BOB'S BLUE MATT Chrome Oxide
CoO Cobalt Carbonate
BRINGLE'S GREEN Copper Carbonate
Cr2O3
BUTTERMILK Custer Feldspar
CuO Dolomite
DEPENDABLE RED FeO EPK Kaolin
F-4 Feldspar
GAIL'S WHITE K2O Ferro 3134
LAURA'S TURQUOISE Li2O Gerstley Borate
Lithium Carbonate
MYSTERY BLUE MgO Magnesium Carbonate
Na2O Nepheline Syenite
PIER BLACK OM-4 Ball Clay
RACHEL'S BLUE P2O5 Red Iron Oxide
SiO2 Rutile
RUTILE Silica
SnO2 Strontium Carbonate
SEAFOAM Talc
SrO
SPECKLED LAVENDER Tenn #10 Ball Clay
TiO2 Tin Oxide
TENMOKU ZnO Ultrox (Zircopax)
Whiting
WOO BROWN TO BLUE ZrO2 Zinc Oxide
YELLOW SALT
12. Clays Bentonite
EPK (Kaolin)
Primary source of OM-4 Ball Clay
Aluminum Tennessee #10 Ball Clay
Feldspars Custer Feldspar
F-4 Feldspar
Primary source of Li, Nepheline Syenite
Na, and K, and also Spodumene
provide Silica and
Aluminum
13. Soda Ash
Fluxes Lithium Carbonate
Strontium Carbonate
Strontium Oxide
Sources of Lithium, Magnesium Carbonate
Sodium, Magnesium, Dolomite
Calcium and Strontium Talc
Whiting
14. Borax
Glass makers Ferro 3134
Sources of Boron, Gerstley Borate
Silica and Silica
Phosphorus Bone Ash
Rutile
Opacifiers Tin Oxide
Sources of Titanium, Zinc Oxide
Tin, Zinc and Zirconium Ultrox (Zircopax)
15. Chrome Oxide
Colorants Cobalt Carbonate
Copper Carbonate
Red Iron Oxide
Black Iron Oxide
16. Carbonates vs. Oxides or Silicates
• Carbonates • Oxides or Silicates
– Easier to make fine – Can be coarse and heavy
powders and hard to disperse
– Lower density and easier – No gas production
to suspend in glaze – Less toxic because they
– Produce large amounts are harder to absorb
of CO2 – bubbles – Higher density means
– Generally more toxic less material required
Cobalt Carbonate vs. Cobalt Oxide
Calcium Carbonate (whiting) vs. Wollastonite (CaSiO4)
17. Some of these compounds are toxic
Lithium , Copper, Cobalt and Chromium
are elements of concern
Eat a spoonful of these and you will get sick or die
We need small amounts in our diet to be healthy
(1mg/d Li, 2mg/d Cu, 0.1 mg/d Co, 10mg/d Cr)
18. Chronic inhalation of dust
• Regular inhalation of fine silica dust causes long term
health problems
• Fine silica powder is a mainstay of both clays and
glazes
• This is the number one health
concern in the studio
19. Keep Dust Levels Low !
• Clean up drips and spills
• Don’t dry sweep
• Don’t dry sand inside the studio
• If you have an apron, wash it regularly
• If you have a towel, keep it damp, wash it
• Transfer large bags of powder outside
• …
23. Melting Mixtures
• Many of the glaze
compounds melt at very
high temperatures
• Mixtures of these
compounds melt at
lower temperatures
(Eutectic = easy to melt)
25. The first objects in the solar
system were ceramic
The small white
inclusions are
Ca-Al-silicates
We can study their compositions and compare to known phase
diagrams to infer the conditions under which they formed
26. On an atomic scale our ingredients
are big
• Melting begins at the
contact points
• Our “small” particles are
still tens of thousands of
atoms wide
• This affects how the
glaze melts
27. Kilns
Electric Gas or Wood
• Oxygen atmosphere • Combustion gas atmosphere
• Precise temperature • Moderate temperature
control control
• Generally below 2250F • Routine operation to 2350F
(cone 6) to increase
filament life but can go to (cone 10)
cone 10 • Capable of producing
reduction atmosphere
• Reduction difficult • Difficult to control
28. Combustion
• CH4 + 2O2 CO2 + 2H2O (890 J/mole)
– All the oxygen is consumed
– Maximum heat production
• CH4 + O2 CO + H2 + H2O (36 J/mole)
– Not enough oxygen for complete combustion
– Most of the fuel energy escapes
– Large amount of carbon monoxide produced
29. There are many possibilities
• CH4 +yO2 aCO2 + bCO +cH2 + dH2O + eC
• Even molecules like CH3OH (methanol)
• Details matter - burner geometry, kiln size
• Gas kilns differ greatly
30. Why care about reduction ?
• CO + 2CuO Cu2O + CO2
• Color changes in the
transition metal colorants
31. Timing in the firing matters
• Before the glaze melts, the glaze and clay are
porous and interact with the kiln atmosphere
• Once the glaze melts, the interaction with the
kiln atmosphere takes place by diffusion –
much slower
32. Timing in cooling matters
• Once the burners are off, the atmosphere
suddenly changes to being oxygen rich
• As the glaze cools, some liquids or solids by
come out of solution
• Crystal formation is critical to copper red
glazes
• Crystal formation and phase separation give
rise to many of the effects we like
33. Safety in firing the gas kiln
• Carbon Monoxide – very toxic
• You can hurt your eyes looking in the kiln
• Volatile compounds
– organic, sulfurous, metallic compounds
Gases coming from the kiln, especially during reduction
are dangerous. Stay away from the kiln during firing.
Beware of the possibility of kiln exhaust entering the
studio
34. Evaluating Glaze Compositions
• Lots of different ingredients bring in some of
the same oxides and it can be hard to
compare glaze recipes
• We need to be able to transform glaze recipes
into lists of basic oxides
• Instead of using weights, it is useful to
calculate relative number of molecules
35. Calculating Molecular Fractions
• Need to know the composition of each ingredient and what
disappears during firing
Custer Feldspar OM #4 Ball Clay Calcium Carbonate
(whiting)
CaO 0.30 CaO 0.30
K2O 10.28 K2O 1.00 CaO 56.10
Na2O 2.91 MgO 0.40
Al2O3 17.35 Na2O 0.30 LOI 43.90
SiO2 69.00 TiO2 1.20
Fe2O3 0.12 Al2O3 27.90
SiO2 55.20
LOI 0.04 Fe2O3 1.10
LOI 12.60
Values are weight % LOI = Loss On Ignition
37. Seger Unity Formula
Seger Formula: Normalize the mole % values to the sum of the fluxes
Oxide Mole % Seger Oxide Mole % Seger
CaO 0.1480 0.61 SiO2 0.6500 2.678
K2O 0.0561 0.23 Al2O3 0.1062 0.438
MgO 0.0373 0.15 TiO2 0.0011 0.004
Na2O 0.0004 0.00
Fe2O3 0.0009 0.00
sum 0.2427
The Seger formula for a cone 6 cone has SiO2 = 6,
a cone 7 cone has SiO2 = 7…
38. Mixing Glazes
• Read and understand the recipe
• Check to see if you have the ingredients
• Equipment:
– A scale to weigh materials
– Containers to weigh materials and mix materials
– Sieves to do the final mixing
– Graduated cylinder and funnel to measure specific
gravity
– Protective equipment, dusk mask, gloves …
39. What I do to mix a glaze
1) Start with 0.5 liter of water per 1000g of dry material
2) Start with the hard to mix materials first
1) Bentonite, ball clay, EPK
3) Mix each ingredient as you add it
4) Add water to bring the specific gravity close to the
correct value so the glaze isn’t too thick but still needs a
little more water
5) Take notes (and keep them) of what you did
6) Take a break (30 min – a day) for material to hydrate – it
will be easier to sieve and mix
40. What I do to mix a glaze
7) Spend several minutes with the drill mixer to thoroughly
mix the glaze
8) Sieve the glaze a couple time (80 mesh, check recipe)
9) Check the specific gravity and add water to bring it to
the correct value (about 1.6 – check recipe)
10) Check the thickness (viscosity). Test how it coats a
piece of bisque pottery (want about 1mm)
11) If the glaze is thin, add epsom salt (MgSO4)
(20g/10000g glaze) at a time until the glaze is thickened
– no more than 100g/10000g total
12) Clean up the mess
41. Measuring Specific Gravity
• Specific gravity is the measured density
divided by the density of water
• Weigh 100 ml of glaze and divide the weight
by 100
• Measuring specific gravity is the best way to
know how much water to add
42.
43.
44. Glaze Flocculation
• Glazes are complex liquids – a suspension of fine particles
• Clay particles have interesting surface properties and
depending on what’s dissolved in the glaze, they will stick
together
• Dissolved Ca and Mg will cause glazes containing clay to
flocculated
• A flocculated glaze works well for dipping application
• If a glaze doesn’t contain much clay, bentonite can be
added (1-2%)
• Some glazes will de-flocculate over time due to materials
going into solution. Add small amounts of epsom salt
(MgSO4) to re-flocculate the glaze
46. Measuring Glaze Thickness
• Applying glaze at the proper thickness is
important
• Know the glaze and measure the thickness
Use a razor blade to scrape a 90 degree
scratch in the glaze. The width at the
top is twice the thickness of the glaze