Alum — the double sulfate salts (potassium aluminum sulfate and related compounds)
Alum refers to potassium aluminum sulfate and a family of double sulfate salts. This article explains composition, common types, history, practical uses, safety and how alums differ from related aluminum compounds.
Overview
Alum commonly denotes potassium aluminum sulfate, but the term also identifies a broader class of isostructural double sulfate salts. In everyday usage, “alum” may mean the specific compound KAl(SO4)2·12H2O or any similar salt in which a monovalent cation (like K+ or NH4+) pairs with a trivalent cation (such as Al3+ or Cr3+) and sulfate groups to form a hydrated crystal. For a concise reference to the compound, see the entry often labeled specific alum.
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Structure and common types
Alums share a characteristic formula and hydrate: M+M3+(SO4)2·12H2O, where M+ is a singly charged cation and M3+ is a trivalent metal. They crystallize in a cubic lattice and typically form octahedral or dodecahedral crystals. Common examples include:
- Potassium alum (KAl(SO4)2·12H2O), often called simply “alum” and historically important in dyeing and tanning.
- Ammonium alum (NH4Al(SO4)2·12H2O), used in similar applications and in some laboratory procedures.
- Chrome alum (KCr(SO4)2·12H2O), where chromium replaces aluminum and alters color and properties.
For the specific chemical identity of potassium aluminum sulfate, consult sources labeled potassium alum.
History and origin
Alums have been used since antiquity for tanning leather, fixing dyes to textiles and as clarifying agents. The name derives from the Latin alumen, meaning a bitter salt. In medieval and early modern Europe, alum trade and production were economically significant because the compound was essential to the textile industry.
Uses and practical importance
Alums serve several roles across industries and household uses:
- Textile dyeing and printing, where an alum acts as a mordant to bind dyes to fibers.
- Water treatment and clarification, owing to coagulating properties that help settle suspended particles.
- Cosmetic and medicinal uses: small amounts are used in some deodorants, as a mild astringent or styptic for minor cuts.
- Culinary and baking applications have historically used related aluminum-containing leavening agents, though modern food chemistry often prefers specific additives with regulated purity.
Technical discussions and material data are available through specialist resources identified as double sulfate references.
Properties, safety and distinctions
Alums are typically water-soluble, forming clear, hydrated crystals. They are chemically distinct from simple aluminum sulfate or alumina; “alum” implies the double-salt structure and specific hydration level. Many alum compounds are low in acute toxicity at small quantities but can irritate skin or mucous membranes; ingestion of large amounts or chronic exposure to aluminum compounds is a subject of health and environmental study, so usage follows safety guidance and regulatory limits. When choosing an alum for a task, consider the specific cations involved and applicable safety data sheets.
History
The Egyptians already used alum as a flame retardant for wood. Pieces found date back to 450 B.C. The Romans also used it for this purpose and additionally mixed it with vinegar. Furthermore, they used it as a deodorant: "It removes the stench under the armpits as well as sweat".
Alum was also used in alchemy around the 11th century.
During the 14th and 15th centuries, the Medici family of Florence had exclusive rights to this mineral, which was used to dye cloth. Together with the church, they operated the first European alum works in Tolfa.
In 1578 Horatio Palavicino, a banker who had converted to Protestantism and whose family administered the papal alum monopoly, sold the entire stock to the Dutch in exchange for a promissory note signed by Queen Elizabeth of England for £29,000 and the granting of the import monopoly. England had long suffered from the fact that the monopoly then rested with the Papal States, and imports, which were important to the cloth industry for fixing dyes, were hampered after the Church broke away from England under Henry VIII. Prospecting was then done at home and Thomas Chaloner set up his own alum industry in Yorkshire from 1607 based on alum slate.
In the 1830s, Leopold Bleibtreu (1777-1839) and his brother Abraham (1775-1852) operated the largest alum smelting company in Prussia in what is now the Holzlar district of Bonn.
Usage
Alum is used in tanning to whiten hides, in calico printing and in fabric dyeing for mordanting (see also dyeing plants). It is also used for waterproofing fabrics, which are then drawn through oleic acid, for clarifying liquids, and so on. In many cases the alum must be entirely free from iron, the presence of which is detected by means of blood liquor salt (blue dyeing). In papermaking, dyeing, and white tanning, aluminium sulphate itself is now frequently used instead of alum, and is therefore often called concentrated alum.
- Ammonium alum (Alumen amoniacale), (NH4)Al(SO4)2-12 H2O, is prepared in the same way as potash alum, by adding ammonium sulphate to aluminium sulphate instead of potassium sulphate. It contains 49.62% water of crystallization, is more readily soluble in cold water than the ordinary potash alum, and is used like the latter. The content of anhydrous aluminium sulphate in potash alum is 10.8 %, in ammonium alum 11.9 % and in so-called concentrated alum (see above) 15.4 %. Ammonia alum, which, incidentally, is often a mixture with potash alum, emits the pungent odour of ammonia (ammonia solution) when treated with caustic potash.
- Sodium alum is rarely used because it quickly weathers, becomes cloudy, and eventually decomposes to a white powder.
- Chromalaun, KCr(SO4)2-12 H2O (Alumen chromicum), in which the aluminium is replaced by chromium, is used in dyeing and tanning. It is often formed as a waste product in the manufacture of tar dye, in which chromic acid is used as an oxidizing agent, and consists of octahedra which are almost black in incident light and dark red in transmitted light, and which dissolve in water with a violet colour.
Alum is also used to make plasticine. The most common application in daily life is the styptic pencil, which is used as an astringent to stop bleeding. Alums were already used in medicine in ancient times and the Middle Ages. But alum is also used in gardening. Here hydrangeas are fertilized with alum to produce a violet or blue coloration of the flowers. In Thailand, for example, it is added to water to bind the suspended matter in it and thus clarify the earthy water. It is also used as a deodorant (French Pierre d'Alun). Alum is wetted and applied to the areas to be deodorized. In China, alum powder (Chinese 明矾粉, pinyin míngfánfěn) has been used for centuries to make the fried breakfast pastry youtiao (Chinese 油条, pinyin yóutiáo).
- Crystal growers often use potassium aluminum alum and potassium chromium alum. Both alums can be grown into centimeter-sized crystals.
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AlegsaOnline.com Alum — the double sulfate salts (potassium aluminum sulfate and related compounds) Leandro Alegsa
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