Pottery and Ceramics Tools

Kiln Firing Temperatures

How Different Temperatures Effect Clay

212 °F

Water of plasticity evaporates, unbound water and the evolution of steam.

842 °F

Chemically combined water is given off from the evolution of steam.
Other types of clays loose chemically combined water between 842° F and 1112° F, and there is minor shrinkage and increase in porosity.

932° F

Hydrocarbons from organic matter begin to burn out, blackening the surface as they are driven off.

1063° F

Quartz changes structure and expands by 2% by volume.

1112° F to 1472° F

Salts have fallen out of the narrow spaces between the particles and they melt to form a liquid film between clay and softened particles.
Surface diffusion and vapor transport of clay parts begins, resulting in necking.
No, it doesn't mean that the ceramics and pottery is kissing and hugging in a hot kiln, click on the word and it will tell you what is happening for sure!!

1598° F

decomposition of limestone

1472° F to 1652 °F

Clays react with fluxes, such as potassium carbonate, to form a viscous silicate liquid in the narrow spaces of the particles.

1652 °F

Volume diffusion occurs, causing further necking, shrinkage, densification.
The crystal structure of most clays is lost, resulting in high temperature shrinkage.

1922° F to 2192° F

Feldspars begin to melt and dissolve silica and some other materials creating a glassy phase of the ceramic, pores close and there is a rapid decrease in porosity.

Earthenware

1112° F to 1472° F

Soluble impurities concentrate at particle contact points during drying and during firing these contact points become liquid at this temperature.
Capillary pressure glues the particles together resulting in little shrinkage, earthenwares 15 to 30 volume percent porosity remain from original 30% to 50%.

Changes below show the microstructure of an iron containing earthernware shard.

1292° F

There is little change from its appearance before firing, but liquid has formed, attaching the particles together to provide a useful permeable product.

1472° F

Reaction produces a mixture of crystalline particles in a glassy matrix, forming a harder less permeable ware.

1652° F

The ware consists entirely of a viscous liquid that will barely hold its shape.
Higher firing temperatures would cause slumping or bloating.

1472° F to 1652° F

This is a critical range. Potassium carbonate is present and decomposes at 1472° F, but doesn’t react until reaching a higher temperature.
Clay and fluxes react to form viscous silicate liquid and crystalline material.

Temperature Considerations

The illustration below shows bodies formulated for successful firing over a wide range of time and temperatures.
These bodies have refractory grains dispersed in a viscous silicate liquid containing an extensive network of fine crystallites that stiffen the liquid and prevent excessive slumping.

Rate of Temperature Increase

Slow enough to allow escape of gases, expansion or contraction accompanying structural and chemical changes.
Fast enough to maximize fuel economy.

As Temperature Increases

Chemical changes and liquid formation increases, viscosity decreases and fluidity increases.
Advantage of liquid formation is vitrification, densification, lower porosity.
Disadvantage of liquid formation is capillary pressure decreases and possibility of slumping increases.

Rate of Temperature Decrease

Fast decrease for glassy liquid to form by reaction of clay, temper and flux which will form solid glass.
Slow decrease so that crystals may form from glassy liquid as it is cooled, which is a partial crystallization.

As temperature decreases, shrinkage or volume expansion start, which is different for ceramic body and glazes.

Atmosphere considerations

Fuel and Air Ratio

Surplus oxygen, oxidizing atmosphere.
Restricted air supply and damp fuel, reducing atmosphere.
Role of Iron Oxide in Sintering

Oxidizing

Red hematite, ferric oxide
1.5 oxygen and iron atom

Reducing

Black magnetite, ferrous oxide forms above 1562° F, 1.33 oxygen and iron atom.
Magnetite forms more liquid at 1472° F to 1652° F.
Reduction results in harder ware.

Putting Iron Oxide to Work

Greek black on red wares.
In Greece, illite is a common clay type.
Slips suspend illitic clays in a liquid and add iron oxide.
This results in a fine particle size alkaline rich iron containing clay.
Suspending this clay in some water makes a slip.
Cover the clay body with the slip in the areas intended to be black.

Fire ceramic in reduction.

Burnished slip has more alkali and forms a black glassy layer and the body is gray.
Open kiln, cool in air and a porous body forms red hematite.
The impermeable glassy slip remains black.

Firing Oooooop's

Shattering or Blowing

Rapid generation of steam, ware not dry enough or temperature increased too fast.

Spalling

Cracking, spit out or lime popping due to high of temperature, to prevent fire under 1292º F.

Warping

Uneven contraction upon cooling or uneven distribution of heat in kiln.

Sagging

Firing to a temperature too high.

Firing Shrinkage Excessive

Addition of temper reduces firing shrinkage as well as lower contraction on cooling. Quartz and lime increase cooling contraction, increasing alkaline glaze fit compatibility.

Particles Too Large

Cracks form along temper particles during firing.

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