Mackenzie Cupola
The Encyclopedia of Founding (1892)
The Mackenzie cupola has a continuous tuyere, which allows the blast to enter the fuel at all points. Each size is proportioned to melt a given quantity of iron in a certain time. Above one ton per hour melting capacity they are made oval in form. This construction brings the blast to the centre of the furnace with the least possible resistance and, it is claimed, the smallest amount of power, causing a complete diffusion of air and a uniform temperature. The sizes of the cupolas indicate the melting capacity per hour; that is, a No. 1 melts 1 ton per hour; No. 6, 6 tons; and so on with all sizes. See CUPOLA; BLOWER. Magnesium is a very brilliant metal of almost silvery whiteness. It is more brittle than silver at an ordinary temperature, but becomes malleable at something below a red heat. Its specific gravity is 1.74. It melts at a bright-red heat, and volatilizes at nearly the same temperature as zinc. In dry air its lustre is retained, but a crust of magnesia forms on its surface when subjected to a moist air. Magnesium in the form of a wire or ribbon takes fire at a red heat, burning with a dazzling bluish-white light. The pure oxide of magnesium (magnesia) is obtained by igniting the carbonate, but this is both a difficult and expensive means of obtaining it, and recourse is had to the impure magnesian limestones found in Thuringia and in some parts of England. It is a white powder, varying in density according to the source from whence it is obtained. It is unalterable by heat, and has never been fused; and on account of its refractoriness is valuable as an ingredient for the manufacture of crucibles for metallurgical purposes. See DOLOMITE.
What Has Changed in the Last Century
The 1892 Encyclopedia of Founding treats the Mackenzie cupola as a live commercial product, rated by the ton of iron melted per hour, with proprietary continuous tuyeres and oval shells above one ton. Cupolas themselves survive in iron foundries, now usually water-cooled, often divided-blast or oxygen-enriched and fed with a hot blast, but the named proprietary designs of the 1890s are long gone and most modern melting for iron and steel castings is done in electric induction or arc furnaces rather than coke-fired cupolas. The magnesium note appended to the same page has dated further: the metal melts at about 650 degrees C, is now produced on a large scale by electrolysis of magnesium chloride and by thermal reduction rather than being a laboratory curiosity, and its chief use is in light structural alloys instead of the dazzling flash-light of Victorian photography. The claim that magnesia "has never been fused" no longer holds; fused magnesia has been made in electric furnaces since the early twentieth century, magnesium oxide melting near 2,800 degrees C.
Written for this edition, 2026.