Editing Sand casting (section)

===Green sand===
{{further|Molding sand#Green sand}}
These castings are made using sand molds formed from "wet" sand which contains water and organic bonding compounds, typically referred to as clay. The name "green sand" comes from the fact that the sand mold is not "set", it is still in the "green" or uncured state even when the metal is poured in the mold. Green sand is not green in color, but "green" in the sense that it is used in a wet state (akin to green wood). [[Misnomer|Contrary to what the name suggests]], "green sand" is not a type of sand on its own (that is, not [[greensand]] in the geologic sense), but is rather a mixture of:

* [[silica]] sand (SiO<sub>2</sub>), [[chromite]] sand (FeCr<sub>2</sub>O<sub>4</sub>), or [[zircon]] sand (ZrSiO<sub>4</sub>), 75 to 85%, sometimes with a proportion of [[olivine]], [[staurolite]], or [[graphite]].
* [[bentonite]] ([[clay]]), 5 to 11%
* water, 2 to 4%
* inert [[sludge]] 3 to 5%
* [[anthracite]] (0 to 1%)

There are many recipes for the proportion of clay, but they all strike different balances between moldability, surface finish, and ability of the hot molten metal to [[degasification|degas]]. Coal, typically referred to in [[foundry|foundries]] as [[sea-coal]], which is present at a ratio of less than 5%, partially combusts in the presence of the molten metal, leading to offgassing of organic vapors. Green sand casting for non-ferrous metals does not use coal additives, since the [[carbon monoxide|CO]] created does not prevent oxidation. Green sand for aluminum typically uses [[olivine]] sand (a mixture of the minerals [[forsterite]] and [[fayalite]], which is made by crushing [[dunite]] rock).

The choice of sand has a lot to do with the temperature at which the metal is poured. At the temperatures that copper and iron are poured, the clay is inactivated by the heat, in that the [[montmorillonite]] is converted to [[illite]], which is a non-expanding clay. Most foundries do not have the very expensive equipment to remove the burned out clay and substitute new clay, so instead, those that pour iron typically work with silica sand that is inexpensive compared to the other sands. As the clay is burned out, newly mixed sand is added and some of the old sand is discarded or recycled into other uses. Silica is the least desirable of the sands, since metamorphic grains of silica sand have a tendency to explode to form sub-micron sized particles when thermally shocked during pouring of the molds. These particles enter the air of the work area and can lead to [[silicosis]] in the workers. Iron foundries expend considerable effort on aggressive dust collection to capture this fine silica. Various types of respiratory-protective equipment are also used in foundries.<ref>{{Cite journal|title=Respiratory Health and Cross-Shift Changes of Foundry Workers in Iran|author1=Mayam Saraei |author2=Habibbolah Masoudi |author3=Omid Aminian |author4=Nazanin Izadi |journal=Tanaffos Journal of Respiratory Diseases, Thoracic Surgery, Intensive Care and Tuberculosis|year=2018|volume=17|issue=4|pages=285–290|pmid=31143220|pmc=6534795}}</ref><ref>{{Cite journal|url=https://www.foundrymag.com/opinion/article/21926109/respirator-use-and-practices-in-primary-metal-operations|title=Respirator Use and Practices in Primary Metal Operations|journal=Foundry Management and Technology|access-date=2021-04-05}}</ref>
    
The sand also has the dimensional instability associated with the conversion of [[quartz]] from alpha quartz to beta quartz at 680&nbsp;°C (1250&nbsp;°F). Often, combustible additives such as wood flour are added to create spaces for the grains to expand without deforming the mold. [[Olivine]], [[chromite]], etc. are therefore used because they do not have a [[phase transition]] that causes rapid expansion of the grains. Olivine and chromite also offer greater density, which cools the metal faster, thereby producing finer grain structures in the metal. Since they are not [[metamorphic minerals]], they do not have the [[polycrystals]] found in [[silica]], and subsequently they do not form hazardous sub-micron sized particles.