< INTRODUCTION TO PATTERNS

DIFFERENT TYPES OF MASTER PATTERN

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

LOST PATTERNS

Also known as DIRECT CASTING, lost pattern casting is the oldest wax based casting technique. 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].

Most famously, during the 15th century, Renaissance, masters such as Ghiberti, Brunelleschi and Cellini (to name just two), revitalised the technique after examining the methods practiced by their Greek and Roman predecessors. 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 quite different points in 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 directly used to create the cast. As a concequence of this, the original sculpture is always destroyed (lost) during the casting process. To enable the artist’s pattern to be directly used in the casting process, the original sculpture must be formed from a material that is capable of being flushed out of a heat resistant mould. This feature restricts the artist to modelling their work in easily degraded materials like wax. When Ghiberti and Brunelleschi submitted their respective designs for the doors of the Florentine Baptistery, they both modelled in wax, not clay; use of this modelling material enabled each artist to produce a direct cast metal representation [ref 3 ].

There are both advantages and disadvantages in using the lost pattern method of casting. One of the main advantages of this process is it’s directness – no REPORDUCTION MOULD (rubber) is needed to produce a metal cast of the original sculpture. A small lost pattern can therefore be taken to the foundry, have a running assembly attached, be invested in a refractory material, burnt out and cast in fairly short order.

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 first being that it is not possible to produce any further copies of the original. 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 could possibly be lost completely. Such an event would mean that a completely new pattern design would have to be re-modelled and then re-cast.

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. Cores are discussed in more detail in the WAXWORKING section of the site, however it is worth briefly outlining their purpose here.

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 surface FAULTS and excessive shrinkage (significant contracton of the cast’s expected dimensions), both of which are detrimental to the overall quality of the cast.

It can often be difficult for the founder to insert a core into a lost wax pattern after the design has been completed in the artist’s studio. Even if the artist’s design has been hollowed out to receive a core, the weight of the added refractory material can easily distort, and even collapse work constructed in an insubstantial material like wax. For this reason most modern lost pattern casts are carried out a small scale sculptures only, usually artist’s wax sketches which are rarely much bigger than hand size.

If a large lost pattern cast is going to be attempted, the standard solution is to first create a core which closely follows the intended form of the artist’s design, then work the (wax) modelling material over the core’s surface. This technique usually means the artist first has to construct an internal metal armature to support the weight of core. The armature must be made with a high degree of accuracy, because if any part of the framework lies too near to the surface of the core, the metal construction might expand near a weak point when in the kiln, damaging the investment mould. Once the armature is fixed, the artist can build up core material over the metal framework, just as they would for a solid modelled plaster construction. The core material itself should be mixed to the founder’s specifications using approved refractory materials. Most core materials for lost wax casts consist of PLASTER and FIRECLAY GROG, this is prepared for use and manipulated in more or less the same manner as plaster of Paris.

The optimum thickness of modelled wax over the 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 dictates 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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© Robert Moule 2008