Alum (English: Alum, French: Alun, Latin: alum ) was formerly only the crystallized hydrous sulphate double salt of potassium and aluminum ( aluminum potassium sulfate) called. Recently, one called so sometimes the corresponding ammonium salt of aluminum while the name alums applies to all sulfuric acid double compounds of similar chemical constitution, in which case the entering of potassium or aluminum metal of the name is prefixed, for example alum for the sulphate double salt of potassium and chromium. Alums always have the composition Mimiii (SO4) 2.12 H2O wherein MI monovalent metal cations, such as the alkali metals sodium, potassium, rubidium and cesium, - with the exception of lithium, since it can not be incorporated without loss of stability - thallium, or ammonium, and its can be organically substituted derivatives; MIII can be the following three times positively charged metal cations: aluminum, gallium, indium, titanium, vanadium, chromium, manganese, iron, cobalt, rhodium, iridium and partly thallium. Typical alums are the aluminum, chromium and iron. All alums crystallize in the cubic system, usually in the octahedral and always with 12 molecules of water of crystallization. From the sodium only two very fickle alums are also aware of the aluminum and the chrome - alum.


The Egyptians used alum already as flame retardants for wood. Found pieces date from the year 450 BC The Romans used it for this purpose and offset it with additional vinegar. Furthermore, they used it as a deodorant, " he removed the stench under the armpits as well as the sweat ."

During the 14th and 15th century, the Florentine Medici family had the sole right to compensation this necessary for cloth manufacture mineral. It operated together with the Church, the first European Alaunwerk in Tolfa.

1578 sold the Protestant convert banker Horatio Palavicino, whose family ruled the papal Alaunmonopol, the entire stock against one of Queen Elizabeth of England signed promissory note for 29,000 pounds and the granting of the import monopoly of the Dutch.

In the 1830s operated Leopold Bleibtreu ( 1777-1839 ) and his brother Abraham ( 1775-1852 ), the largest Alaunhüttenunternehmung Prussia in the present district of Bonn Holzlar.


For the preparation of potash alum light burned, iron-free as possible tone with 50% sulfuric acid in the heat treated. This forms the separation of silica aluminum sulfate ( historically: sulphate of alumina).

After dilution with water and removal of the insoluble residue, the silicic acid, potassium sulfate or chloride is added. That which thereby separates as a fine powder " Alaunmehl " is purified by recrystallization from hot water. Analogously, bauxite or cryolite are used.

Production in Alaunwerken (19th century): In some areas, such as Schwemsal in today's Saxony-Anhalt (near Bad Duben in Saxony ), alum and leached from roasted alum was prepared. In this case the roasted iron sulphide provided the aluminum sulfate required to form sulfuric acid and the clay minerals aluminum. Potassium was added mostly as potassium carbonate ( K2CO3). The removal of the iron is effected by suitable choice of the concentration of the solution.

In Tolfa in Italy and in Munkács in the (former Hungarian ) western Ukraine, the mineral Alum or alunite occurs, a basic potassium aluminum sulfate. This material was roasted and leached with hot water to form insoluble alumina remained, while all components of the alum went into solution and separated out on cooling in usually by finely divided iron oxide slightly reddish crystals.


The alum has a lot of use, with almost always occurs only aluminum sulfate in effectiveness. In the manufacture of paper he used in the sizing with (modified) tree resins as precipitant for this, in the tanning of the skins to Weißgarmachen, in calico-printing and in dyeing stuff for pickling (see also dye plants ). It also is used for waterproofing of stuff substances that are then pulled through oleic acid, for clarification of liquids, etc. In many cases, the alum must be completely free of iron, whose presence is detected by potassium ferrocyanide ( blue staining). In papermaking, dyeing and white tannery, now often used instead of alum aluminum sulfate itself, which is therefore often referred to as " concentrated alum ".

  • Ammonium alum ( alum amoniacale ), ( NH4) Al (SO4) 2.12 H2O, corresponding to the potassium alum is produced by admixing aluminum sulfate instead of potassium sulfate using ammonium sulfate. It contains 49.62 % water of crystallization, is easily soluble in cold water than the ordinary alum, and is used like this. The content of anhydrous aluminum sulfate is at 10.8% Potash alum, ammonium alum at 11.9% and the so-called concentrated alum ( above) 15.4%. Ammoniakalaun, which often is actually a mixture with potassium alum, when treated with caustic potash are the pungent smell of ammonia ( ammonia ).
  • Natronalaun is rarely used because it weathers quickly, becomes cloudy and eventually disintegrates into a white powder.
  • Alum, KCr (SO4) 2:12 H2O ( alum CHROMICUM ), in which the aluminum is replaced by chromium, is used in dyeing and tanning. He often arises as a waste product in the Teerfarbenherstellung in which the chromic acid is used as an oxidizing agent, and is made by reflected light almost black, dark red by transmitted light octahedra, which dissolve in water with a violet color.

Alum is also used to make plasticine. Frequent use in daily life is the styptic pencil, which is used as an astringent to stop bleeding. But even in the garden essence alum is employed. Here hydrangeas are " fertilized " with alum to produce a violet or blue color of the flowers. In Thailand, it will supply the water added to bind the suspended therein and to clarify the water so earthy. Also available as deodorant (french Pierre d' Alun ) it is made. Alum is wetted and coated on to the deodorizing places. Alum closes unlike the conventional deodorants not pores but prevents odor bacteria from multiplying, such that for several hours odor is suppressed. In China, for the production of fried breakfast pastries Youtiao (Chinese油条, Pinyin yóutiáo ) been used for centuries powdered alum (Pinyin Chinese míngfánfěn明矾 粉, ) used.

  • Also popular are the potassium - aluminum and potassium chrome alum in crystal growers. Both alums can be very beautiful breed to centimeter-sized crystals.

Potash alum

The potash alum, KAl (SO4) 2.12 H2O, contains 45.5 % water of crystallization, forms colorless, transparent, sometimes very large crystals, which cover only on prolonged storage in air with a fine, white opaque film of weathered substance tastes bitter and sweet at the same time and may be harmful in larger doses. The specific gravity is given by Kopp with 1,924. On heating the alum first melts in its water of crystallization, and finally leaves a loose, white, opaque mass, which is called burnt alum ( alum ustum ). Its solubility is very big in hot water, very low, so that the preparation of pure succeed relatively easily in cold; the solution is acidic. The most common crystal form shows the octahedron, where sometimes the cube faces occur deadening. Especially this is in the past because of its purity, highly valued Roman alum of the case, which was also called cubic or Würfelalaun. The cube shape can be achieved by adding to the hot concentrated solution of potash ( potassium carbonate), soda ( sodium carbonate) or ammonia, adds that until a permanent precipitate is formed, and then allowed to cool first. Such crystals are called neutral or blunted alum and are, in chemical terms, basic salts.


Formula: KCr (SO4) 2.12 H2O ( full name chromium ( III) potassium sulfate dodecahydrate or potassium chromium ( III) sulphate ). Alum is dark purple and crystallized in octahedrons. Mixed with potassium alum, it is well suited for simple crystal growth.

  • Aluminum alum single crystal of chromium doped

The same crystal with a view of the triangular surfaces ...

And ... from above.

Alums with selenates

There are also a few selenates SeO42, which correspond to the alums in their crystal structure. So far, those compounds known exclusively with the large cesium ion as an alkali component. The general formula is CsMIII ( SeO4 ) 2:12 H2O ( MIII = Al, Ga, Fe, In, Cr, Rh).

List of alum combinations


  • NaAl (SO4) 2.12 H2O
  • KAl (SO4) 2:12 H2O
  • RbAl (SO4) 2:12 H2O
  • CsAl (SO4) 2:12 H2O
  • TlAl (SO4) 2:12 H2O
  • NH4Al (SO4) 2:12 H2O


  • KGa (SO4) 2:12 H2O
  • RBGA (SO4) 2:12 H2O
  • CSGA (SO4) 2:12 H2O
  • TlGa (SO4) 2:12 H2O
  • NH4Ga (SO4) 2:12 H2O


  • RBIN (SO4) 2:12 H2O
  • CS in (SO4) 2:12 H2O
  • NH4In (SO4) 2.12 H2O


  • KT (SO4) 2:12 H2O
  • RBTI (SO4) 2:12 H2O
  • CSTI (SO4) 2:12 H2O
  • NH4Ti (SO4) 2:12 H2O


  • KV (SO4) 2:12 H2O
  • RBV (SO4) 2.12 H2O
  • CSV (SO4) 2:12 H2O
  • NH4V (SO4) 2:12 H2O


  • NACR (SO4) 2:12 H2O
  • KCr (SO4) 2:12 H2O
  • RbCr (SO4) 2:12 H2O
  • CSCR (SO4) 2:12 H2O
  • TLCR (SO4) 2.12 H2O
  • NH4Cr (SO4) 2:12 H2O


  • RbMn (SO4) 2:12 H2O
  • CSMN (SO4) 2:12 H2O


  • KFe (SO4) 2:12 H2O
  • RbFe (SO4) 2:12 H2O
  • CSFE (SO4) 2:12 H2O
  • TlFe (SO4) 2:12 H2O
  • NH4Fe (SO4) 2:12 H2O


  • KCo (SO4) 2:12 H2O
  • RBCO (SO4) 2:12 H2O
  • CSCO (SO4) 2:12 H2O
  • NH4Co (SO4) 2:12 H2O


  • RbRh (SO4) 2:12 H2O
  • CsRh (SO4) 2:12 H2O


  • RBIR (SO4) 2.12 H2O
  • CSIR (SO4) 2:12 H2O


  • Adolf Beythien, Ernst Dressler (ed.): Merck goods lexicon. 7th edition. Gloeckner, Leipzig 1920 ( unaltered reprint: Manuscriptum, Recklinghausen 1996) Merck's goods encyclopedia
  • August Nies: Alum. In: Pauly Realencyclopädie of classical archeology (RE). Volume I, 1, Stuttgart 1893, Sp 1296 f