Qualities of refractories used in ancient copper smelting clearly affected furnace operation and smelting efficiency. Severe slag attack around the tuyeres is characteristic of the ancient furnaces dated to the late Bronze/early Iron Ages at Timna, Israel. The ancient tapped slag compositions are also relatively consistent. Experimental smelting furnaces built from mixtures of clay and sand exhibited similar low resistance to the fayalite-type slag compositions. The emphasis of the reconstruction smelting experiments was to identify appropriate methods of furnace control. A mass balance between copper ore and flux was unachievable under ancient conditions. Extensive furnace slagging could not be prevented. Assuming the use of excess flux, the furnace lining provided a reservoir of silica, in part, to balance slag composition. Thus, practical effects of chemical and physical slag-refractory reactions were observed, seemingly exploited, and to a degree successfully controlled.