Production
Information
Methods of shaping ceramic piggy banks
- On the potter’s wheel: This is modelling a piggy bank like a pot on a potter’s wheel. Potters are not too keen on making piggy banks this way.
- Free modelling: ‘Free modelling’ is an artistic method. The results are fantastic, but very few potters do it this way.
- Moulding: Most frequently used is moulding. Inside the mould you find the ‘negative’ image of the piggy bank. Slip clay is poured into the mould, water is soaked up in the absorbent mould. All is left to dry for a small period and then the rest of the slip clay is poured out again. To pour out the slip a piggy bank needs to have a hole. That could one of the reasons modern piggy banks have stoppers. Makers of old fashioned piggy banks used creative means to let the slip go: a hole where the tail of head later was put on, or through one or more legs. They still have to cut off the remaining “funnels”. This filling opening has the length of the thickness of the mould. The traditional opening for the coins is cut in the raw material before firing. See my blog (2015/12/01).
- Folding: An obsolete method because of the labour intensity. Nowadays only used by potters on fairs. See my blog called "Folding" (2018/01/01).
Producing ceramic piggy banks
- Preparing clay: It all starts with preparing the clay. The clay is kneaded to remove air bubbles and get an even consistency.
- Shaping: The clay is then shaped. This can be done by hand, on a turntable, or using moulds. Each piece takes its basic shape in this phase.
- Drying: The shaped piggy bank must dry before it can be baked. This may take several days depending on the size and thickness of the piece.
- Engobe layer: After drying, a thin layer of clay sludge (engobe) is sprayed onto the piggy bank.
- Baking (first time): The dried piggy bank is baked for the first time in an oven. This is called the 'biscuit bake' and is done at a lower temperature (approximately 900-1160°C).
- Painting: often done by hand. After painting, the glaze layer is applied.
- Glazing: After the biscuit tray, the piggy bank is glazed. Glaze is a glass-like layer that makes the piggy bank waterproof and gives it a beautiful finish.
- Baking (second time): The glazed piggy bank is fired again, this time at a higher temperature (about 1200-1300°C) and remains in the oven for about 24 hours. This melts the glaze and makes the piggy bank strong and durable.
Transfer technique
The transfer technique on ceramics is special. With extra steps in the process that ensure that the decorations are well protected and the end product has a beautiful, durable finish.
- In this technique, images or patterns are printed on special paper and then transferred to the glazed piggy bank. This happens after the first glaze layer and the second firing.
- Second layer of glaze: After applying the transfers, a second layer of glaze is often applied. This could be tin glaze, for example, which gives a beautiful, bright finish and protects the transfers.
- Third Bake: The piggy bank is fired again to melt the second glaze and make the transfers permanent. This second bake is usually done at a lower temperature than the first bake to prevent the transfers from being damaged.
Shrinkage
- In traditional wood- or coal-fired ovens, the temperature distribution can be uneven, leading to variations in shrinkage depending on position in the oven. This is because the heat source is not evenly distributed, causing some parts of the oven to be hotter than others. Products that are closer to the heat source may shrink more than products that are further away.
- In modern gas-fired ovens, the temperature distribution is much more controllable. Gas ovens often have multiple burners and advanced temperature controls that ensure even heat distribution throughout the oven. This results in more consistent shrinkage and fewer dimensional deviations between products coming from the same mold.
So, with traditional (now long since demolished) ovens you could get piggy banks from the same mould that differ in length, while with modern gas-fired ovens this is much less likely. This is also an identifier for the age of a piggy bank.
Terms connected with earthenware
- 'Bisque' of 'biscuit' earthenware is simply the once-fired body without the addition of glaze. Green ware is unfired pottery, ready to be bisque fired.
- Terracotta: Unglazed porous earthenware, mostly red.
- Slip (liquid clay) is used in casting and to decorate earthenware.
- Carving: Like woodcarving removing areas of clay in the raw model, to create a raised design. After carving the object is baked and then glazed.
- Plaster: Used to make moulds but was also used (especially in France) to create figurines and piggy banks. The process is the same as clay slip in moulds. There is no baking process, the plaster dries in the air and is painted after drying.
- Stoneware: It became known in northern Europe after the Renaissance. Probably the majority of current glazed stoneware are salt glazed (part of the baking process).
- Siderolith: White or coloured volcanic clay, baked as stoneware. But Siderolith does not form a glass layer on top as stoneware does. It is finished with copper varnish. Production between 1880 and 1930. Much wanted by collectors!
- Glaze: A thin coating of glass. An impervious silicate coating, which develops in clay ware by the fusion under heat of inorganic materials. Applied to porcelain or pottery body to make it waterproof and enhance colour. Glazes may be clear, opaque or coloured to various degrees.
- Lead glaze: transparent glassy glaze using lead oxide, colour green. Due to EC regulations forbidden for household use, because it is poisonous.
- Tin-based glaze is white, glossy and opaque, normally applied to red or buff earthenware. The opacity and whiteness of tin glaze make it valued by the ability to decorate it with colour.
- Painting: paint over tin-based glaze or plaster.
- Lustre: A type of decoration originally developed in Persia that leaves a thin layer of metal on the decorated portions of pottery.
