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Lathe Machining of Bronze Coin Flans
 
Continued (p.2)

 The flans could be cast in permanent molds made of plaster or cut into stone, or alternatively wax patterns could be formed in permanent molds and then flans could be prepared by a lost-wax casting process. The lost-wax or investment casting process allows casting of fine detail, but is laborious since the wax patterns must be coated by a refractory slurry which can then be baked or fired into a hard shell, during which the wax melts or vaporizes.  

In either case, the periphery of the flan would have pieces of the runner left when the flans were cut apart. Also, it was (and still remains) common practice to put an “overflow” runner on the opposite side of the flan as illustrated in Figure 3.  The “overflow” allowed the molten metal to flow through the flan so that the “skin” of oxides and slag formed at the leading edge of the molten metal stream would not become part of the flan. Thus, many flans actually had two runners, a feed runner and an overflow runner. The metal in the sprues and runners was cut up and remelted after being detached from the flans. 


Fig. 3
Overflow runner 

Molten metal poured into a mold  does not form a homogeneous structure since, as the metal cools, alloy phases with the highest melting point solidify first. This causes variations in metal composition between the center of a flan and its surface. The metal in contact with the surfaces of the mold solidifies first, while the metal at the center of the thickest features of the casting solidifies last. Molten metal shrinks considerably as it solidifies, and to avoid formation of voids, it is important to ensure that there is a reservoir of molten metal in the runner that can move into the casting as the metal solidifies. The desired solidification sequence is: Overflow and its runner first, casting second, and the feed runner last. It is often useful to keep the area of the feed runner heated to ensure this. 

Metal in contact with the surface of the mold normally forms a skin whose composition and properties are somewhat different from the metal at the center of the casting. The copper-tin alloy phase diagram is very complex (see Figure 4). However, it is only the very left side of this diagram that concerns us because the tin content of bronze coins is normally less than 15% by weight. In this range the phase structure is simple: the alpha phase (up to 15% Sn) begins to solidify at about 1080°C, and as the temperature falls to between 800°C  and 900°C solidification becomes complete. When bronze is cast, its surface (particularly in alloys with high tin content) tends to become hardened. A metal is harder when it has small grains, which result when the metal is cooled rapidly, and this chilling occurs at the surface of the casting, particularly when metal is poured into a mold that is at ambient temperature. Because of the manner in which the molten metal moves into the mold, the casting surface also may contain various imperfections such as oxides, slag, air and gas bubbles, cold shuts and other solidification flaws, and even bits of material from the matrix in which the casting is made.  


Fig. 4
Cu – Sn Alloy Phase Diagram 

For these reasons the surface layer or skin of the cast flan was harder, and more likely to cause abrasion due to slag, oxide and matrix inclusions, than the more homogenous metal below the surface of the flan. The surface skin would also be more likely to cause appearance flaws in the finished coin due to the persistence of surface casting irregularities during striking.

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