Chrome Flour

General Information

How to Make Green Glass

Chromite is an oxide mineral which is naturally found below ground in deposits, the world’s most known and highest quality supply is found in southern Africa. The mineral chromite is used as a tool in the glass manufacturing industry, it provides for better heat tolerances and in some cases may behave in a flux like manner.

Unlike reduced green glass, which has a positive redox number, oxidised glasses have negative redox numbers, they retain higher levels of sulphate and are associated with traditional green coloured glass, such as Emerald Greens which mostly possesses a redox number of -5 and is made using around 0.2 wt% chromate and 0.5 wt% iron oxides and Georgia Greens which is a light green, almost a blue-ish shade of glass associated with Coca-Cola bottles. This colour is formed due to the ratio of chromite and iron oxide highly in favour of the iron, with quantities of chromite in the region of 0.05 wt%. Georgia green glass typically has a dominant wavelength at circa 555 nm. Dead leaf - or feuille morte - is achieved when twice the amount of iron oxides is used compared to chromite.

Unlike oxidised green glass, reduced green glass have positive redox numbers, they retain low levels of sulphate and are associated with ‘UVA glass’ which is a yellow-green glass that is resistant to ultraviolet radiation which glassmakers are especially attracted to, as items contained within UVA glass are shielded from radiation which may cause them damage. UVA glass is significantly less of a green colour than that of emerald green glass, due to the presence of small quantities of stabilised hexavalent chromium, which lessen the green colour from chromium(iii) using its own yellow. As little as 0.1 wt% of chrome oxides by mass is sufficient to produce UVA green, the contribution of iron oxides is in the region of 0.4 to 1 wt%

Chromium has been used in various compounds as a container glass oxide since the early 19^th century. It was discovered in the Russian mineral crocoite in 1795. Crocoite is a lead chromate. The name chromium comes from the Latin “chromos,” meaning colour, indicating the striking colours of chromium compounds.

Before chrome flour 325 and its compounds were discovered, iron oxide or a combination of cobalt and yellow were used to make green glass, though it is believed possible that the unknown presence of chromium is responsible for the green colouration in some cases. Sand for glassmaking has been found to be contaminated with chromium in both England and the United States.

The compounds used for glass are derived from the trivalent and hexavalent chromium. African Pegmatite’s chrome is the more stable and environmentally-friendly trivalent Cr2O3. Hexavalent chrome is a hazardous carcinogen that is heavily regulated. Cr2O3 is “practically immiscible” according to EN Bunting, Bur. Standards J. Research 1930, and has a low solubility in silicate glasses with a low or normal alkali content. Lithia glasses have been found to be better solvents for chromite powder than soda or potash glasses.

To produce bright emerald green without a yellow cast, it is necessary to add reducing agents. H Jebsen-Marwedel reported on iron chromite powder as a container glass colorant. He found that a potassium chromate solution sprayed over the batch had the lowest occurrence of defects when using a low concentration of chromium. For higher concentrations of the green glass oxide, a sifted chromium ore was found to be suitable.

A speckling or swirling effect known as the aventurine effect can happen when the temperature is too low for sufficient solubility. This happens because the green glass oxide separates from the glass in thin hexagonal plates.

Iron chromite can form compounds with acids in colours from blue and green to dark red. The exact colour will depend on hydration, crystalline structure, and size. Changes in lighting and thickness can cause distinctive changes in the colour of some chromic salts, which appear green when thin or viewed in daylight and appear red when thick or viewed under an incandescent lamp. This can also be seen in alexandrite, which contains chromium.

For this reason, iron chromite powder can’t be used alone to colour signal glass such as traffic lights and boat or aeroplane signal lights. These glasses require copper oxide as a colourant, with chromium possibly used as a modifier.

The first recorded study of chromium flour in glass took place in 1901. R Zsigmondy tested glasses with chrome flour 325 under the same oxidising conditions. No significant differences were found in light absorption of the base glasses.

In 1923, K Fuwa expanded on this study by investigating the effects of oxidation and reducing variables. He found that the introduction of chromate or dichromate produced a yellowish green while the introduction of the trivalent chromium we offer as a container glass pigment produced a range of emerald greens, particularly in the presence of reducing agents. Fuwa concluded that chromium might exist in both the trivalent and hexavalent states.

Zsigmondy also discovered that chromite powder in boric-oxide glasses produced darker colours. By increasing the amount of boric oxide in the container glass pigment gradually, he was able to create a range of greens that reached olive green at 15% to 20%. This was achieved with the hexavalent chromite flour.