An Ancient Beauty Aid That Keeps On Giving

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The purpose of bisk firing is (1) to completely vitrify or densify a ceramic body in the case of bone china, fine china, and industrial ceramics or (2) to partially vitrify a body in the case of porcelain.
Taking porcelain first, Case (2), the body is taken to a temperature to which enough strength is developed to be automatically handled in glazing operations (in modern factories) but porous enough to be easily glazed by dipping methods.
Porcelain wear does not have to be supported during bisk firing and defects generated are usually chips or breakage from handling.
Kiln dirt can cause defects here, but rarely.
If they are formed, they often be removed by grinding methods.
Sometimes sand, perhaps alumina, is placed between dinner plates so they can be stacked without any chance of sticking.
This sand is easily removed by subsequent operations.
If a speck of sand is left on the wear during glazing, it will cause a glost ware defect as described in the next article.
In the first case, Case (1), the body is heated on a temperature / time cycle where full strength is developed (usually through sintering) in non-glazed industrial ceramics.
For fine china and bone china, the body develops maximum translucency through vitrification.
The wear often has to be supported during firing.
Three major problems relate to the high-temperature firings of Case (2).
The first problem is shrinkage.
The second problem is slump.
The third problem is drag.
These factors play a major part in the distortion of large parts during firing.
For fine china and bone china, the dinner plates and similar items are fired in refractory saggers.
The ware settles into the shape of the sagger as it softens during firing.
Therefore, the ware is fully supported.
The saggers are covered so there is no kiln contamination.
Operations may be required to remove any particles from the ware before glazing.
Small parts are not a problem for industrial ceramics.
Larger parts are a different matter.
The shrinkage and slump and drag factors must be known.
Shrinkage is caused as the particles move together during sintering or vitrification.
Slump is due to the weight of the part on itself.
Drag is due to the friction between the part and its setter or the kiln car deck.
A shrinkage block of a body can be formed into a two-diameter cylinder about 3 inches high with a top half diameter of perhaps 1-3/4 inches and a bottom diameter of 2 inches.
The drag shrinkage is determined by the diameter of the block touching the kiln car deck or setter, the slump is determined by measuring the total and shoulder height of the block, and the normal shrinkage is determined by measuring diameters not affected by drag or slump.
An example of how large cylindrical shapes are fired will illustrate what sometimes must be done.
A setter is machined from the same material as the cylinder.
This base setter is coated with a solution of PVA (polyvinyl alcohol) and sprinkled with sand (alumina).
For smaller cylinders this step can be omitted.
On top of this is placed a step setter.
The step is about 3/8 inch.
The ceramic cylinder has its ends coated with a solution of PVA (polyvinyl alcohol), dipped in setting sand (perhaps alumina), and is placed on the step setter.
The upper step is the diameter of the ceramic cylinder.
(Well leave a few thousandths.
) The step setter maintains the lower diameter of the part.
As the base setter shrinks, the base setter (and step setter) will shrink, pulling the cylinder in with it.
But we are not firing yet.
A hollow cylinder (perhaps 2 inches in diameter) made of the same material as the part is placed on the step setter dead center.
A top step setter, treated as above, is placed on top of the ceramic part, its minor diameter fitting into the cylinder and resting on top of the inner support cylinder.
The support cylinder will decrease slump caused by the weight of the top step setter.
The top step setter maintains the diameter of the part.
After firing, the cylinder will not be perfect, but if there is enough grind stock and the upper and lower diameter have been maintained during firing, the part can be machined to the specified dimensions.
I'm not able to illustrate this in this article.
I hope you see it in your mind: A larger cylinder resting on a step setter with a step setter on top, the steps maintaining diameter as they fit into the cylinder.
An inner cylinder supports the top step setter.
Got it? For some shapes and very large cylinders, a refractory solid support cylinder is used.
In this case, the support is placed right on the kiln deck.
A hole is cut in the base and bottom step setters to accommodate the refractory shape.
To protect shapes from shrinkage, drag, and slump, you must design the firing fixtures for every affected part.
Sometimes bubble alumina comes in handy.
Aluminum silicate wool such as Fiberfrax® can be useful for support at fine china bisk temperatures and lower.
There are a thousand chums, setters, and tricks used in the industry.
New ones are invented every day.
For a discussion of ceramic kilns go to Ceramics: Industrial Processing and Testing, John T.
Jones and M.
F.
Berard.
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