< THE BRUSH ON METHOD
SLUSHING
Slushing is a technique regularly used in art foundries to obtain hollow wax patterns.The slushing process partly relies upon the manner in which wax passes through a transitional stage between the SOLIDUS [solid state] and LIQUIDUS [liquid state]. This transitonal phase happens because a typical foundry wax is made up of a number of quite unrelated substances (typically hydrocarbons, esters, fatty acids, alcohols and so on). Each component of the wax product has it’s own molecular weight and consequentially, it’s own melting point. As a result of this variable composition, wax does not suddenly change from a solid to a liquid when heated, or liquid to solid when cooled, but instead passes through a transitional phase - at which point some of the wax's constituents are solid, and others still fluid. This feature explains why a wax in transition often displays a ‘slushy’ consistency. As we have seen, founders can modify a wax blend to vary the temperature levels between which a wax product is fully solid and fully fluid, This allow the founder to create a wax with optimum working properties [ref].
Foundry waxes typically have a temperature transition range which falls roughly between 95°F/35°C (fully solidified) and 194°F/90°C (fully fluid). Though the potential temperature range of a blended product will of course vary considerably according to it’s precise composition.
Waxes for slushing reproduction moulds in the foundry are typically blended to have a relatively low solidification temperature and a long melting range. This effectively means that the wax maintains a fair degree of fluidity at about 110-150°F (43-65°C), or to put it graphically, a temperature more or less cool enough to briefly dip a finger into the wax vat without suffering a burn, (though we suggest you use a thermometer). At these temperatures, the slushing wax can be poured into a reproduction mould without melting and washing away the first ‘painted on’ layer of FACING wax. The cooling wax will also deposit a thickness in the mould reasonably quickly, but not so quickly that the wax build up is difficult to control.
The wax used for slushing a mould may be slightly different in colour to the first painted on layer (or sometimes just lighter in tone). A lighter tone or different colour in the slushed wax clearly shows against the darker painted up layer, and helps indicate mould seams for retouching later. The colour difference can also act as an alert, helping to identify any areas of the wax pattern that are excessively thin (see WAX CHASING).
The slushing process first requires the melting of an appropriate quantity of wax in a container with enough volume to more or less fill at least 50% the mould. Melting wax from it’s solid state requires the wax to be taken to a far higher temperature than desirable for slushing, so once fully fluid the wax is removed from the heat source and allowed to cool down to a more suitable level. The temperature of the cooling wax is either controlled automatically by thermostat, or monitored by thermometer. Some skilled waxworkers can judge the readiness of the wax by eye; the usual indication here being the formation of a light skin over the surface of the contained wax. Ideally, a wax should be heated, stabilised and held at a constant working temperature (a process known as TEMPERING), for some hours prior to use; though in practice this is not always possible given the demands of a typical working day.
Once an optimum temperature is attained throughout the wax (occasional stirring can help disperse ‘hot spots’), it can be gently decanted into the prepared reproduction mould. The accurate pouring of the wax into a mould may be assisted either by the use of a temporary clay funnel which is formed around the mould’s entrance, or else through a preformed plaster pouring cap, secured by DOGS to the outer mould case. As the wax is being decanted into the mould, care is taken not to disrupt the painted on first layer, at the same time avoiding the creation of trapped air pockets. Any subsidiary vents from the mould’s interior to the outside are sealed with clay as the wax level rises. As the mould fills with fluid wax it is agitated or tapped softly with a mallet to disturb any trapped air. Dependent upon the founder’s preferences, the mould cavity is either filled to it’s funnel entrance and allowed to stand, or else only partially filled and then rotated to evenly distribute the wax across the rubber’s surface, expelling any excess material before it cools.
If a poured wax is allowed to stand in the mould it gradually solidifies, first where the wax is in contact with the mould’s rubber surfaces, continuing to freeze inwards towards the centre of the filled mould cavity. In this way a skin layer of increasing thickness is formed in the mould. If left unattended, the contained wax volume in the mould would eventually set into a solid mass. Because wax deposits first where it contacts the mould’s rubber surface, the founder can arrest the ever increasing wall thickness at any suitable moment, simply by evacuating the fluid wax from the mould cavity. Expelling the excess wax from the mould will leave an evenly distributed thickness of wax over the surface of the mould to form a hollow wax pattern.
Timing, and the use of a suitable wax blend are critical in obtaining good results with this ‘rested’ method of slushing moulds. A wax with either too short a melting range or poured too cold, is likely to freeze rapidly creating a potentially heavy cast. Too long a melting range, or too hot a pouring temperature results in the waxworker having to attend moulds for over-extended periods of time. Much of the first ‘painted on’ layer is also likely to be washed away by an excessively hot slush, and the loss of this layer may result in a poorly reproduced surface in the wax pattern. These drawbacks mean that many founders find the alternative technique of rotating and manipulating a mould just partially filled with fluid wax offers better control, despite being more physically demanding.
WAX INJECTION >
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