< INTRODUCTION TO PATTERNS

DIFFERENT TYPES OF MASTER PATTERN

There are two basic types of master pattern that are used for lost wax casting, LOST PATTERNS and PERMANENT PATTERNS. The distinctions between these two kinds of master pattern can appear to be quite marginal, however the underlying differences are quite marked and worth keeping in mind.

LOST PATTERNS

Also known as DIRECT CASTING, lost pattern casting is the oldest of the wax based casting techniques. The LOST PATTERN method was used to produce many past masterpieces of the founders craft – Uruk seals in Mesopotamia (3,500 bc), Egyptian images of deities (from about 2200 bc), and later, various Greek, Eutruscan and Roman depictions and cast vessels. Direct casting methods were also practiced outside of Europe and the Near East – in the New World of South and Central America, West Africa (Ashanti, Benin and Igbu Ukwu peoples), and in Sub-continental India (Chola), during the 9th and 10th centuries [ref 1].

During the 15th century, Renaissance, masters such as Ghiberti, Brunelleschi and Cellini revisited the lost pattern technique after studying the methods practiced by the Greeks and Romans. In fact, the lost pattern process is universal enough to have probably been independently developed and practiced by a number of disparate cultures and civilisations, at very different points in man's history [ref 2].

WHAT IS A LOST PATTERN?

The lost pattern process is so named because the artist’s original sculpture (the MASTER PATTERN), is always destroyed (hence 'lost') during the casting process. For the artist’s pattern to be directly used in the casting process this way, the pattern must be formed from a material that is capable of being flushed out of a heat resistant mould. This need restricts the artist to modelling their work in an easily degraded material like wax. When Ghiberti and Brunelleschi submitted their respective relief designs for the doors of the Florentine Baptistery, they both modelled in wax (not clay); the use of wax enabled each artist to produce a direct cast metal of their submission [ref 3 ].

ADVANTAGES & DISADVANTAGES

There are of course advantages and disadvantages in using the lost pattern method of casting. One of the main advantages of this process is it’s directness – for example no REPORDUCTION MOULD (rubber) is needed to produce a wax version of a sculptural design (unlike designs created from clay or metal for instance). A small lost pattern can therefore be taken to the foundry, have a metal running assembly attached, be invested in a refractory material, burnt out and then cast in a relatively short time frame.

The main disadvantage of the lost pattern method is the inevitable destruction of the artist’s original sculptural design. This has a number of important implications, the most obvious being that it is not possible to produce any further copies from the original design. The founder could later take a rubber reproduction mould from the metal cast (a technique known a SURMOULAGE), however any copy produced via surmoulage would always be slightly smaller than the original due to reproduction materials shrinkage. Secondly, in the event of anything going seriously amiss during the casting process, the artist’s work would probably be lost completely; this would mean that a new pattern would have to be modelled from scratch and re-cast.

CORES

A further disadvantage of the lost pattern method is that any large scale volumetric sculpture must have some provision made for a CORE to be inserted, otherwise the produced cast will always be solid. A core consists of a solid refractory mass, which once set inside a wax  pattern, enables a small air gap to be created between the core and the outer investment mould after the wax design has been melted away. This air gap allows molten metal to be run through the mould, where it then freezes to create a thin walled hollow cast in the image of the wax design. If a core is not placed within a wax design, the poured metal would fill the entire cavity of the investment mould and solidify as a heavy mass. Apart from being wasteful of metal, large volumes of solid metal are often associated with major surface faults and excessive shrinkage in castings, both of which can be very detrimental to the overall quality of a cast artwork.

If a large scale lost pattern cast is going to be attempted, the usual solution is to first create a core which closely follows the contours of the artist’s design. The artist then models a wax skin over the underlying core’s surface. The core material itself should be prepared to the founder’s specifications using their preferred refractory materials. Most core materials for lost wax casts consist of PLASTER and FIRECLAY GROG, this core material can usually be mixed and worked in more or less the same way as plaster of Paris.

The optimum thickness of modelled wax over a core will vary according to the scale of the sculpture, however in most cases an average of 3/16 in (4-5 mm) will suffice. When modelling wax over a core it is essential to keep a close eye on thickness variations, this is because the wax layer determines the wall thickness of the metal cast. Too heavy a wax layer may cause surface faults and lead to an overweight cast, too thin a wax layer and metal may  fail to run through and fill the investment mould.

MATERIALS FOR LOST PATTERNS >

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