Didymium
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.)
Didymium photographic filters are often used to enhance fall scenery by making leaves appear more vibrant.  This is accomplished via the special properties of the substance which causes the removal of part of the orange region of the color spectrum. When present, this group of colors tends to make certain elements of a picture appear "muddy".
Didymium is also used in calibration materials.

# 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 Jons 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 Bastnas, Sweden.  Mosander may have named "didymium" with a bit of tongue-in-cheek: an allusion to the fact that his four children had been born as pairs of twins!  Didymium had not been difficult to find, since it was providing the pinkish tinge to the salts of 1803 ceria when in trivalent form.  The real tour de force was to find the colorless lanthanum in the mixture! 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.
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.  
Recently, it was found that praseodymium could be included in the neodymium-iron-boron magnet composition without excessive sacrifice in magnetic strength.  This has the effect of "stretching" the neodymium supply, while simultaneously finding a home for the under-utilized praseodymium.  For such magnets, the appropriate mixture can be prepared directly by solvent extraction without purifying either component separately.
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