Didymium (Greek: twin element) is a mixture of the elements praseodymium and neodymium. It is used in safety glasses for glassblowing and blacksmithing, especially when a gas (propane) powered forge is used, where it provides a filter which blocks the yellowish light emitted by the hot sodium in the glass, without having a detrimental effect on general vision, unlike dark welder's glasses. Blocked also is the strong ultraviolet light emitted by the superheated forge gases and insulation lining the forge walls thereby saving the crafters' eyes from serious cumulative damage. (See also arc eye, also known as welder's flash or photokeratitis.)

History

Didymium was discovered by Carl Mosander in 1841 and was so named because it is very similar to lanthanum, with which it was found. Mosander wrongly believed didymium to be an element, under the impression that "ceria" (sometimes called cerite) isolated by Jöns Jakob Berzelius in 1803 was really a mixture of cerium, lanthanum and didymium. He was right about lanthanum's being an element, but not about didymium. Mosander did as well as could be expected at the time, since spectroscopy had not yet been invented. His three "elements" accounted for at least 95% of the rare earths in the original cerite from Bastnäs, Sweden. Didymium had not been difficult to find, since it was providing the pinkish tinge to the salts of 1803 ceria when in trivalent form. During the period when didymium was believed to be an element, the symbol Di was used for it.

In 1874 Per Teodor Cleve concluded that didymium was composed from two elements.In 1885 Carl Auer von Welsbach succeeded in separating salts of these elements, today known as praseodymium and neodymium, by a fractional crystallization of the double ammonium nitrates from nitric acid. He wanted to name his two elements "praseodidymium" and "neodidymium" ("green didymium" and "new didymium", respectively), but a syllable was soon dropped from each. The "didymium" name lived on in untruncated version, in part due to the use in glassblower's goggles. The "didymium" name also lived on in mineralogical texts.

During World War I, didymium glass was reputedly used to send Morse Code across the battlefields. Didymium did not absorb enough light to make the variation in lamp intensity obvious, but anyone with binoculars attached to a prism could see the absorption bands flash on or off.

The name lived on also in the rare earth industry. In the USA, commercial "didymium" salts were what remained after cerium was removed from the natural abundance mixture obtained from monazite, and thus contained lanthanum as well as Mosander's "didymium". A typical composition might have been 46% La, 34% Nd, 11% Pr, remainder mostly Sm and Gd, for material extracted from South African "rock" monazite (from Steenkampskraal). European usage was closer to the Mosander composition. Such cerium-depleted light lanthanide mixtures have been widely used to make petroleum-cracking catalysts. The actual ratio of praseodymium to neodymium varies somewhat depending on the source, but is often around 1:3. Neodymium always dominates, which is why it got the "neo" appellation, being responsible for most of the color of the old "element" in its salts. Typically, in ores, neodymium is higher in relative abundance in monazite, as compared to the bastnaesite compositions, and the difference is noticeable when unseparated mixtures derived from each are examined side-by-side: the monazite-derived products are more pinkish, and the bastnaesite-derived products are more brownish in tinge, due to the latter's increased relative praseodymium content. (The original cerite from Bastnaes has a rare earth composition highly similar to that of monazite sand.)

In the late 1920s, Leo Moser recombined praseodymium and neodymium in a 1:1 ratio to create his "Heliolite" glass, which has color-changing properties between amber, reddish, and green depending on the light source. [One can only hope that an appropriate intermediate fraction of the Pr-Nd separation might have been used, to save some expense, since at the time, separated praseodymium and neodymium oxides were the most costly glass colorants in use.]

Usage

Didymium lenses were originally designed for the glass blowing industry. Didymium is used to shield the eyes from dangerous forge and furnace bright yellow light. The lenses are uv protected as well. Once the bright yellow light is removed you can see the surface of metal much clearer with no eye strain.

The lenses do shift the colours a bit but you can get use to that pretty quick. The question I had was can you buy them in a perscription format. The answer is yes but I recommend that instead you buy the clip on version as your perscription may change and you just have to switch the clip ons then.

Your eyes are important! The minimum protection is safety glasses. The auralens product is much better (also makes forge welding easier as you can see the surface of the metal at welding heat).

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