The word dichroic (pronounced dye-kro-ik) comes from the Greek meaning two colours. Dichroic Glass is glass coated with multiple thin film layers of various elements to transmit a particular desired colour, and reflect another.
The technology was developed in Germany over 100 years ago by Dr. Arthur Pfeiffer. Dichroic coatings have several uses. These range from use in diagnosing diseases (by tracing fluorescent antibodies), to use in heat-seeking missiles. The technology, called "Thin Film Physics", has evolved from the U.S. aerospace programs into the glass art field.
The process of making the glass is complex. Firstly the glass is optically cleaned in a meticulously clean room. The slightest speck of dust can mar the coating. The glass is polished with rouge, and then cleaned to the molecular level with several types of solvent alcohol. It is then coated with 15 to 45 extremely thin films of elements, such as titanium, magnesium, zirconium, etc. The elements are evaporated and vacuum deposited onto the glass to such a fine degree of thickness that certain wavelengths of light will pass through and others will be reflected. During the deposition process, the glass is also continuously rotated inside the vacuum chamber, in order to deposit uniform coatings on the glass. However, there are always slight differences in colour throughout the sheet of glass. The whole process time required in the vacuum vessel is from two and a half to four hours, depending on the structure.
This is the reason dichroic glass is so very expensive.
The thin film layers have no colour of their own, but produce colour by acting as an "Interference Filter" (like the colours seen on a soap bubble). This ‘filter' has the unique characteristic of shifting its colour, when viewed from different angles or in varying degrees of illumination. The range of colours depends on the number and type of elements used and their relative positions in the layers applied.
When dichroic glass is used to make jewellery, several transformations take place. As the glass heats up, it expands at a greater rate than the coating. When the material cools and the glass contracts, the coating never gets back to its original position. The higher the temperature, the more severe this affect is.
This is why it is almost impossible to create two pieces that are the same.