| The purpose of bisk firing is (1) to completely | | | | together during sintering or vitrification.Slump is |
| vitrify or densify a ceramic body in the case of | | | | due to the weight of the part on itself.Drag is due |
| bone china, fine china, and industrial ceramics or | | | | to the friction between the part and its setter or |
| (2) to partially vitrify a body in the case of | | | | the kiln car deck.A shrinkage block of a body can |
| porcelain.Taking porcelain first, Case (2), the body | | | | be formed into a two-diameter cylinder about 3 |
| is taken to a temperature to which enough | | | | inches high with a top half diameter of perhaps |
| strength is developed to be automatically handled | | | | 1-3/4 inches and a bottom diameter of 2 inches. |
| in glazing operations (in modern factories) but | | | | The drag shrinkage is determined by the diameter |
| porous enough to be easily glazed by dipping | | | | of the block touching the kiln car deck or setter, |
| methods.Porcelain wear does not have to be | | | | the slump is determined by measuring the total |
| supported during bisk firing and defects generated | | | | and shoulder height of the block, and the normal |
| are usually chips or breakage from handling.Kiln dirt | | | | shrinkage is determined by measuring diameters |
| can cause defects here, but rarely. If they are | | | | not affected by drag or slump.An example of |
| formed, they often be removed by grinding | | | | how large cylindrical shapes are fired will illustrate |
| methods.Sometimes sand, perhaps alumina, is | | | | what sometimes must be done.A setter is |
| placed between dinner plates so they can be | | | | machined from the same material as the cylinder. |
| stacked without any chance of sticking. This sand | | | | This base setter is coated with a solution of PVA |
| is easily removed by subsequent operations. If a | | | | (polyvinyl alcohol) and sprinkled with sand (alumina). |
| speck of sand is left on the wear during glazing, it | | | | For smaller cylinders this step can be omitted.On |
| will cause a glost ware defect as described in the | | | | top of this is placed a step setter. The step is |
| next article.In the first case, Case (1), the body is | | | | about 3/8 inch.The ceramic cylinder has its ends |
| heated on a temperature / time cycle where full | | | | coated with a solution of PVA (polyvinyl alcohol), |
| strength is developed (usually through sintering) in | | | | dipped in setting sand (perhaps alumina), and is |
| non-glazed industrial ceramics.For fine china and | | | | placed on the step setter.The upper step is the |
| bone china, the body develops maximum | | | | diameter of the ceramic cylinder. (Well leave a |
| translucency through vitrification.The wear often | | | | few thousandths.)The step setter maintains the |
| has to be supported during firing.Three major | | | | lower diameter of the part. As the base setter |
| problems relate to the high-temperature firings of | | | | shrinks, the base setter (and step setter) will |
| Case (2). The first problem is shrinkage. The | | | | shrink, pulling the cylinder in with it.But we are not |
| second problem is slump. The third problem is | | | | firing 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 part |
| distortion of large parts during firing.For fine china | | | | is placed on the step setter dead center.A top |
| and bone china, the dinner plates and similar items | | | | step setter, treated as above, is placed on top of |
| are fired in refractory saggers. The ware settles | | | | the ceramic part, its minor diameter fitting into |
| into the shape of the sagger as it softens during | | | | the cylinder and resting on top of the inner |
| firing. Therefore, the ware is fully supported. The | | | | support 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 diameter |
| remove any particles from the ware before | | | | of the part.After firing, the cylinder will not be |
| glazing.Small parts are not a problem for industrial | | | | perfect, but if there is enough grind stock and the |
| ceramics. Larger parts are a different matter.The | | | | upper and lower diameter have been maintained |
| shrinkage and slump and drag factors must be | | | | during firing, the part can be machined to the |
| known.Shrinkage is caused as the particles move | | | | specified dimensions. |