Aluminum sulfate octadecahydrate, \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}${{\rm{Al}}{_{2}}{(} {\rm{SO}}{_{4}}{)}_{3}{\rm{{\cdot{}}}}18{\rm{H}}{_{2}}{\rm{O}}}$\end{document} , and its aqueous solutions are used primarily in the paper industry for sizing and as a flocculating agent in water and wastewater treatment. This material is often called papermakers' alum or alum and is sometimes referred to as cake alum or patent alum. The solid commercial hydrate also called dry alum, ground or lump. Anhydrous aluminum sulfate, Al 2 (SO 4 ) 3 , is a specialty item used in food applications. Over 50 acidic, basic, and neutral aluminum sulfate hydrates have been reported. Commercial dry alum is likely not a single crystalline hydrate, but rather it contains significant amounts of amorphous material. In the United States, aluminum sulfate is usually produced by the reaction of bauxite or clay with sulfuric acid. Unfortunately, the term alum is now used for several different materials. Papermakers' alum or simply alum refers to commercial aluminum sulfate. Common alum or ordinary alum usually refers to potash alum which can be written in the form \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}${{\rm{K}}{_{2}}{\rm{SO}} _{4}{\rm{{\cdot{}}}}{\rm{Al}} _{2}{(}{\rm{SO}} _{4}{)}_{3} {\rm{{\cdot{}}}}24{\rm{H}}{_{2}}{\rm{O}}}$\end{document} , or it can refer to ammonium alum, ammonium aluminum sulfate. The term is also applied to a whole series of crystallized double sulfates having the same crystal structure as the common alums. Some examples of alums are cesium alum, \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}${{\rm{CsAl}}{(}{\rm{SO}}{_{4}}{)}_{2}{\rm{{\cdot{}}}}12{\rm{H}}{_{2}}{\rm{O}}}$\end{document} ; iron alum, \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}${{\rm{KFe}}{(}{\rm{SO}}{_{4}}{)}_{2}{\rm{{\cdot{}}}}12{\rm{H}}{_{2}}{\rm{O}}}$\end{document} ; chrome alum, \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}${{\rm{KCr}}{(}{\rm{SO}}{_{4}}{)}_{2}{\rm{{\cdot{}}}}12{\rm{H}}{_{2}}{\rm{O}}>}$\end{document} ; and chromoselenic alum \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}${{\rm{KCr}}{(}{\rm{SeO}}{_{4}}{)}_{2}{\rm{{\cdot{}}}}12{\rm{H}}{_{2}}{\rm{O}}}$\end{document} . In industrial practice it is generally the aluminum content of alums that is important. Because aluminum sulfate is widely available, other alums are more in the nature of specialty items.