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