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