MELTING AND POURING METALS
WARNING: Take extreme care when working with hot materials, follow OHS guidelines and safe working procedures. Undergo professional training before attempting the processes described in this section.
The procedures used for melting and pouring metals into evacuated refractory moulds varies somewhat according to the metal alloy being used, the type of furnace and the individual preferences of the founder. An outline procedure for the melting and GRAVITY CASTING of a copper based ALLOY (BRONZE, GUNMETAL etc), into an refractory mould using a traditional LIFT OUT type gas or oil fired furnace, is set out below. The procedure for pouring sand and other refractory moulds is essentially the same, bar some minor variations.
INFO: For more detailed metallurgical information on melting metals & alloys see: SOLID TO LIQUID & BACK AGAIN / SOLID TO LIQUID & BACK AGAIN (ALLOYS).
1. The furnace is lit and a removable CRUCIBLE (constructed from a refractory graphite mix), is pre-heated by placing close to the furnace exhaust, or else inside the furnace body with the lid left open.
2. The INGOT for the charge (including an additional margin allowance), is measured out, then placed in close proximity to the furnace exhaust to drive off any ambient moisture. The charge may also partially be made up of previously cast off-cuts, spent runners and cups known as RETURNS. Returns should not exceed 25% of the total charge by weight and are always of the same alloy composition as the virgin ingot. The use of scrap metal of unknown origin is not recommended (note: SILICON BRONZE will tolerate a higher proportion of returns than GUNMETAL).
3. Once the crucible and charge have been sufficiently pre-heated, the fresh ingot and returns are carefully lowered into the crucible, avoiding any hard wedging (known as BRIDGING), against the crucible wall. Bridging can cause the crucible wall to split open as the jammed charge heats up and expands.
4. As the charge melts, a powder COVER FLUX is added into the crucible. The cover flux floats on top of the melted charge and helps prevent the absorption of atmospheric gases that could adversely affect the metal. In the absence of a proprietory cover flux, a small block of hardwood placed under the initial charge at the bottom of the crucible provides a similar fluxing effect. As an additional measure to reduce gas pick-up in the charge, the atmosphere within the furnace body is adjusted to be slightly oxidising, this atmosphere helps exclude hydrogen gas from the charge and gives a copper alloy melt it’s distinctive green tinted exhaust flame.
Melted copper alloys can potentially absorb excess levels of oxygen and hydrogen gases. As the charge begins to solidify in the refractory mould after the pour, the oxygen and hydrogen atoms combine to form water vapour, the resulting steam reaction can lead to severe casting faults. The most common fault observed in such cases is a surface porosity and cratering on the cast’s surface, which can be easily mistaken for a similar defect which is caused by pouring metal into a damp mould. The application of a cover flux and the correct furnace atmosphere greatly reduces the potential for gas induced faults to occur.
5. Any topping up of the initial charge is done as the ingot melts down in the crucible, though care is taken to minimally disturb the cover flux when adding in extra metal. The full charge is left to attain it’s pouring temperature, then held briefly to ensure an even and thorough heating. A typical 200lb copper alloy charge could be expected to take a little over an hour from pre-heat to readiness when melted in a commercially manufactured gas fired furnace, though this timescale very much depends upon the age and efficiency of the furnace.
Adding a pre-heated ingot.
6. The melting point of copper is 1985°F (1085°C), though the copper based alloys commonly used for fine art founding are typically melted to a point between 2000°F (1100°C) and 2200°F (1200°C). The optimum pouring temperature of the alloy is also partially determined by the mould’s size and capacity (ie the anticipated wall thickness of the cast). A lower pouring temperature is usual for thick walled or solid casts, with higher temperatures preferred for thin walled designs. The temperature of the melted alloy can be monitored by a THERMOCOUPLE, though an experienced founder will normally be able to judge the correct moment to pour by observing visual cues in the charge’s colour (As a quick guide, most common copper based casting alloys can be judged ready for casting if a pre-heated steel bar is dipped into the crucible and held for a few seconds; if the bar is removed free of any adhering charge the metal should be hot enough for pouring). Care is taken not to overheat the alloy during the melt, as this can burn off any light alloy content (especially zinc), adversely influencing the quality of the cast (see also METALS).
7. A few minutes before pouring commences, the lifting tongs and shanks which are to be used in hot contact with the crucible or charge, are pre-heated by placing them in proximity to the furnace exhaust. Pre-heating minimises thermal shock and the possibility of either damaging equipment or provoking violent reactions between residual moisture on the tool’s surfaces, and the hot crucible wall.
8. The furnace is shut down, and any temporary cover used to prevent debris from entering the pouring cup is removed. The crucible is lifted clear of the furnace body with scissors tongs, and placed in a specifically designed ring shank. Any cover flux and DROSS (non-metallic residue), is removed off the surface of the standing charge, pulled off with the aid of a steel skimmer.
Skimming dross & lifting.
9. A metallic tube containing an alloy specific degassing agent is plunged into the charge to help purge any residual gases. The crucible can now be secured to the shank ring by a chain or similar device, before lifting either by hand or with mechanical assistance, into position over the pouring cup of the mould.
10. The founder, usually aided by the supporting DEADMAN at the other end of the shank, works to fill the refractory mould as rapidly as possible whilst avoiding undue turbulence in the metal flow, or the creation of back pressure in the mould. An second assistant stands by with the skimmer and prevents any residual dross or oxidised film on the charge’s surface from entering the mould as either of these contaminants could cause surface inclusion faults in the cast. The mould fills up and the pouring cup is topped up to the brim if needs be, thus creating a reservoir of contained metal for the cast to draw upon whilst solidifying. It should also be possible to see metal at the top of any riser outlets. Any surplus metal left in the crucible after pouring is decanted into a pre-heated INGOT MOULD, and allowed to cool before ejection.
Pouring gunmetal into a
plaster & grog mould.
(All images cc ANPP - Fiorini Ltd).
11. The crucible is gently scraped to remove any waste residue, then stood back in the furnace to allow it to cool down gradually, doing this significantly extends it’s working life. Placing a small piece of cardboard on top of the crucible stand before replacement prevents the crucible from sticking to the block when cold.
Poured refractory moulds in the art foundry are normally allowed to cool gradually. With some cast materials (especially glass), this cooling process is extended to include an ANNEALING treatment which might be carried out over a period of some days, gradually reducing the temperature of the cast in a special kiln. Annealing allows the cast material to develop a particular crystalline structure, typically leaving the cast in a more malleable state than a quenched cast. (QUENCHED metals are typically hard and brittle in condition). In most instances, it is desirable to allow art and design casts formed in copper alloys to cool to ambient temperatures in their own time, before removal from the surrounding refractory (no quenching). With other alloys, a rapid quenching of the spent refractory mould and it's contained cast is a preferred method, gold alloys are typical of this.
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