Specular Iron
The Encyclopedia of Founding (1892)
Important: Safety Warning
This page describes trade processes that use dangerously toxic materials such as cyanide solutions, mercury amalgams, or strong acids. These processes require training, equipment, and precautions far beyond what the text describes.
This information has been left in for historical purposes but should not be acted upon.
Specular oxide of iron occurs crystallized in a great variety of forms. Some of these crystals have a polish like burnished steel; others are tarnished and appear of a red, blue, or yellow color. The most beautiful specimens come from Elba, where this iron is said to have been worked for three thousand years. Its composition is iron 69, oxygen 31. Speculum Metals are alloys of exceeding brittleness and hardness, and so brilliant when polished truly as to be used for the mirror-surface of reflecting telescopes. It is therefore called speculum metal. The quality of these alloys greatly deteriorates by a slight deviation to either side of the true atomic proportions. Their extreme brittleness necessitates great care in cooling castings made from them, so that nothing shall interfere to prevent all the parts cooling proportionately. Slow cooling in hot ashes acts beneficially by annealing the metal. As arsenic enters into the most of these mixtures, it is important that a good flux be employed to cause a perfect union with the other metals. One part nitre and two of tartar is a suitable flux for this purpose. The arsenic should be broken in fragments and tied in strong paper; it may then be secured in the tongs and thrust under the surface--after which stir well and avoid the fumes. The following is a table of speculum alloys: SPECULUM ALLOYS. Silver. Copper. Tin. Antimony. Zinc. Arsenic. Brass. Hard, white.... 32 15 2 Hard. White better.... 1 32 15 1 1 Hard. White highly lustrous.... 32 16.5 1.25 4 Hard. White highly lustrous.... 1 32 15 1 Steel, hard.... 1 1 Common.... 2 1 Lord Rosse's.... 126.8 58.9 1 Sir I. Newton's (yellow).... 6 2 1 White (antimony mix.).... 7 1 1 White (with zinc).... 7 4 3 --See ROSSE'S TELESCOPE; BRASS MIRRORS.
What Has Changed in the Last Century
The mineralogy in the 1892 account still holds: specular iron is simply a bright crystalline variety of hematite, and the stated composition of 69 parts iron to 31 of oxygen matches the modern formula Fe2O3. What has gone is everything around it. The Elban mines, worked since Etruscan times, were finally shut down in the 1980s and the island's Rio Marina workings are now a mining park rather than an ore source. And speculum metal, the brittle copper-tin-arsenic alloy tabulated here for telescope mirrors, was already obsolete when the entry was printed: silvered glass mirrors had displaced it from the 1850s, and since the 1930s astronomical mirrors have been glass with a vapour-deposited aluminium coat. The arsenic-handling instructions should be read as history only.
Written for this edition, 2026.