CREATING A FOUNDRY WAX PATTERN

PERMANENT PATTERNS (designs for lost wax casting), constructed in materials like PLASTER and RESIN cannot be directly incorporated into REFRACRTORY [INVESTMENT] MOULDS. The founder instead uses a [rubber] REPRODUCTION MOULD to create a cast wax version of the sculptors original pattern. Wax is used to fill the ‘negative impression’ reproduction mould, solidifying into a positive image reproduction. In the art foundry a wax copy of a sculptor’s design is typically referred to as a WAX PATTERN.

The wax pattern is a near exact copy of the sculptors’s original design (pattern) – near exact because however well made, some of the materials used in this process, including wax and rubber, are subject to a degree of SHRINKAGE. As a result the wax pattern will always be slightly smaller than the master pattern on which it was based. This should not unduly concern the sculptor, because the difference in measurement between the wax and the master pattern is not usually discernible by eye, especially where quality materials are used.

There are a number of methods the founder can use to obtain wax patterns from reproduction moulds. The exact methods used are variable according to the individual preferences of each foundry workshop, and the particular challenges of the job in hand. One of the most common methods is known as SLUSHING.

SLUSHING

Slushing reproduction moulds to create a wax pattern can be briefly surmised as follows: The wax worker first paints a layer of wax into an open reproduction mould, care is taken to ensure all the mould's impressed surfaces are fully coated, air bubbles are forced out and high points (in the mould), are built up slightly more than flatter surfaces (this allows for hot wax 'washing off' material as it passes through the mould). The reproduction mould containing painted wax is reunited and held together with special clamps (DOGS) and/or heavy rubber bands. A suitable quantity of wax is heated and then allowed to cool, usually until it reaches a 'slushy' consistency. The cooling wax is then carefully poured into the mould, which is then rotated and gently tapped to remove any trapped air. The excess wax is then carefully emptied out of the mould and into a container whilst still being rotated. This process deposits an even layer of wax over the surface of the mould, creating a hollow wax copy of the mould's imprinted design. The thickness of wax varies according to the size of the design, however most art foundry waxes are in the region of 3 - 6mm (1/8 - 1/4") thick. This wax thickness eventually becomes the the thickness of cast metal, so getting this stage of the process right is vital.

INFO: Waxes are variously composed of hydrocarbons, esters, fatty acids, alcohols &c. Each of these substances has it's own molecular weight and therefore it's own melting point. This range of melting points means that melting and cooling wax passes through a TRANSITIONAL PHASE where the consolidated wax product is neither fully liquid, nor fully solid ('slushy'). At some point in the transitional stage the wax is at the optimum state of liquidus/solidus to be used for slushing moulds. Metal alloys also often exhibit similar behaviors, these are covered in more detail in the articles: 'SOLID TO LIQUID… ALLOYS' & 'PHASE DIAGRAM A-B' [ref 1]

The founder will usually extract the wax from the surrounding mould and CHASE (work) seams and any other minor faults from the wax pattern's surface. Once this initial preparation process has been completed, most sculptors will then inspect the wax and make any adjustments of their own.

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