Casting processes

Bastian Asmus

A very brief introduction to casting processes

The production and casting of metals is a very old trade and is known at least for the last 7000 years or so. Early evidence for mining can be found at Ai Bunar, Bulgaria and Rudna Glava, Serbia from 5000 BC onwards. Casting is a very efficient and cheap way to manufacture a desired good. This is especially true for metals. Casting processes are can be discerned by the way a mould is produced. There are two possibilities: permanent moulds and lost moulds.
Lost moulds can be used only once. In order to retrieve the raw cast you have to destroy them. Permanent moulds can be used for multiple casts; they are usually made of two, three or more parts, to allow for the retrieval of the cast. Lost moulds require a pattern permanent moulds do not.

Lost moulds

Generally lost moulds are said to be moulds of the lost wax or cire perdu process. This is incorrect as sand moulds are also lost moulds.- With the lost wax-process the patterns are lost also, which requires an individual pattern per cast. Sand moulds on the contrary possess permanent patterns which are being reused every time a sand-mould is made.

Moulding loam – an ingenious material

In general, a moulding material should meet the following requirements:

  • refractory
  • no shrinkage
  • gas-permeable
  • fine-grained
  • slight decay after casting (not relevant for older processes)

Since pure clay is too fat, i.e. it shows too much shrinkage in pure form, the clay must be made leaner. This is done with inorganic and organic materials, which is called temper or grog. Archaeologically proven and/or historically/ethnographically documented temper is e.g.: (quartz) Sand, crushed old forms, animal hair, horse manure and chaff. The clay acts as a binder for the inorganic, refractory ageing agents. Since these do not shrink during drying, the shrinkage of the moulding material can be adjusted via their proportion.

Organic tempers have a different function; they improve the plastic properties in the green state. Due to their fibrous shape, they reduce cracking during drying. They improve the gas permeability as they burn when firing the moulds. The moulding loam is/was produced by each foundryman himself/herself and can be produced with experience at any place where work is/was done. The raw materials are available everywhere.

Historical sources such as Theophilus Presbyter in the 12th century , Vanoccio Biringuccio  or Cellini  in the 16th century share their recipes with us. Also at Lazarus Ercker, the famous sampler and metallurgist of the 16th century, you can find some hints on how to use clay to produce refractory materials .

Dies, permanent moulds

You do not need a model, but you have to engrave a three-dimensional negative into the moulding material. You also want to be careful as to ensure that the cast can be retrieved from the mould halves. This seriously restricts the design of your object and at least for prehistoric times this moulding technique yielded far less complicated cast products than the lost wax process. They are also more labour intensive to manufacture. The advantage is the re-usability of these, and many objects can be cast without much effort once a mould is made. The materials for permanent moulds have to be refractory and hard-wearing, e.g. stone and metal.



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