An alum /ˈæləm/ is a type of chemical compound, usually a hydrated
double sulfate salt of aluminium with the general formula XAl(SO
2O, where X is a monovalent cation such as potassium or ammonium.
By itself, "alum" often refers to potassium alum, with the formula
2O. Other alums are named after the monovalent ion, such as sodium
alum and ammonium alum.
The name "alum" is also used, more generally, for salts with the same
formula and structure, except that aluminium is replaced by another
trivalent metal ion like chromium(III), and/or sulfur is replaced by
other chalcogen like selenium. The most common of these analogs is
chrome alum KCr(SO
In some industries, the name "alum" (or "papermaker's alum") is used
to refer to aluminium sulfate Al
2O. Most industrial flocculation done with "alum" actually uses
aluminium sulfate. In medicine, "alum" may also refer to aluminium
hydroxide gel used as a vaccine adjuvant.
1 Main types
2 Chemical properties
4.1 In antiquity and the Middle Ages
4.2 Modern understanding of the alums
6 Related compounds
6.1 Selenate-containing alums
6.2 Mixed alums
6.3 Other hydrates
6.4 Other double sulfates
7 See also
9 External links
Crystal of potassium alum
Aluminium-based alums are named by the monovalent cation. Unlike the
other alkali metals, lithium does not form alums; a fact attributed to
the small size of its ion.
The most important alums are
Potassium alum, KAl(SO
2O, also called "potash alum" or simply "alum".
Sodium alum, NaAl(SO
2O, also called "soda alum" or "SAS".
Ammonium alum, NH
Aluminium-based alums have a number of common chemical properties.
They are soluble in water, have a sweetish taste, react acid to
litmus, and crystallize in regular octahedra. In alums each metal ion
is surrounded by six water molecules. When heated, they liquefy, and
if the heating is continued, the water of crystallization is driven
off, the salt froths and swells, and at last an amorphous powder
remains. They are astringent and acidic.
Alums crystallize in one of three different crystal structures. These
classes are called α-, β- and γ-alums.
The solubility of the various alums in water varies greatly, sodium
alum being readily soluble in water, while caesium and rubidium alums
are only sparingly soluble. The various solubilities are shown in the
At temperature T, 100 parts water dissolve:
Aluminium-based alums have been used since antiquity, and are still
important in many industrial processes.
The most widely used alum is potassium alum. It was used since
antiquity as a flocculant to clarify turbid liquids, as a mordant in
dying, and in tanning. It is still widely used to purify piped water,
in medicine, for cosmetics (in deodorant and antitranspirants), in
food preparation (in baking powder and pickling), and to fire-proof
paper and cloth.
Sodium alum is used in substitution to potassium alum in baking
Ammonium alum has a few niche uses. Other alums have mostly
In traditional Japanese art, alum and animal glue were dissolved in
water, forming a liquid known as dousa (ja:礬水), and used as an
undercoat for paper sizing.
In antiquity and the Middle Ages
A detailed description of a substance called alumen occurs in Pliny
the Elder's Natural History. By comparing this with the account of
stupteria given by Dioscorides, it is obvious the two are
identical. Pliny informs us that a form of alumen was found naturally
in the earth, and calls it salsugoterrae.
Pliny wrote that different substances were distinguished by the name
of alumen, but they were all characterised by a certain degree of
astringency, and were all employed in dyeing and medicine. Pliny
says that there is another kind of alum that the Greeks call schiston,
and which "splits into filaments of a whitish colour", From the
name schiston and the mode of formation, it appears that this species
was the salt that forms spontaneously on certain salty minerals, as
alum slate and bituminous shale, and consists chiefly of sulfates of
iron and aluminium. One species of alumen was a
liquid, which was apt to be adulterated; but when pure it had the
property of blackening when added to pomegranate juice. This property
seems to characterize a solution of iron sulfate in water; a solution
of ordinary (potassium) alum would possess no such property.
Contamination with iron sulfate was greatly disliked as this darkened
and dulled dye colours. In some places the iron sulfate may have been
lacking, so the salt would be white and would be suitable, according
to Pliny, for dyeing bright colors.
Pliny describes several other species of alumen but it is not clear as
to what these minerals are. The alumen of the ancients then, was not
always potassium alum, not even an alkali aluminum sulfate.
The production of potassium alum from alunite is archaeologically
attested on the island Lesbos. This site was abandoned in the 7th
century but dates back at least to the 2nd century CE.
Native alumen from
Melos appears to have been a mixture mainly of
2O) with potassium alum and other minor sulfates.
The western desert of Egypt was a major source of alum substitutes in
antiquity. These evaporites were mainly FeAl
2O and Al
Alum and green vitriol (iron sulfate) both have sweetish and
astringent taste, and they a had overlapping uses. Therefore, through
the Middle Ages, alchemists and other writers do not seem to have
discriminated the two salts accurately from each other. In the
writings of the alchemists we find the words misy, sory, and
chalcanthum applied to either compound; and the name atramentum
sutorium, which one might expect to belong exclusively to green
vitriol, applied indifferently to both.
Modern understanding of the alums
In the early 1700s,
Georg Ernst Stahl
Georg Ernst Stahl claimed that reacting sulfuric
acid with limestone produced a sort of alum:  The error was
soon corrected by Johann Pott and Andreas Marggraf, who showed that
the precipitate obtained when an alkali is poured into a solution of
alum, namely alumina, is quite different from lime and chalk, and is
one of the ingredients in common clay.
Marggraf also showed that perfect crystals with properties of alum can
be obtained by dissolving alumina in sulfuric acid and adding potash
or ammonia to the concentrated solution. In 1767, Torbern
Bergman observed the need for potassium or ammonium sulfates to
convert aluminium sulfate into alum, while sodium or calcium would not
The composition of common alum was finally determined by Louis
Vauquelin in 1797. As soon as Martin Klaproth discovered the presence
of potassium in leucite and lepidolite, Vauquelin demonstrated
that common alum is a double salt, composed of sulfuric acid, alumina,
and potash. In the same journal volume, Jean-Antoine Chaptal
published the analysis of four different kinds of alum, namely, Roman
alum, Levant alum, British alum and alum manufactured by himself,
confirming Vauquelin's result.
Some alums occur as minerals, the most important being alunite.
The most important alums – potassium, sodium, and ammonium – are
produced industrially. Typical recipes involve combining aluminium
sulfate and the sulfate monovalent cation. The aluminium sulfate
is usually obtained by treating minerals like alum schist, bauxite and
cryolite with sulfuric acid.
Chrome alum crystal
Many trivalent metals are capable of forming alums. The general form
of an alum is XM(SO4)2·nH2O, where X is an alkali metal or ammonium,
M is a trivalent metal, and n often is 12. The most important example
is chrome alum, KCr(SO
2O, a dark violet crystalline double sulfate of chromium and
potassium, was used in tanning.
In general, alums are formed more easily when the alkali metal atom is
larger. This rule was first stated by Locke in 1902, who found
that if a trivalent metal does not form a caesium alum, it neither
will form an alum with any other alkali metal or with ammonium.
Selenium or selenate alums are also known that contain selenium in
place of sulfur in the sulfate anion, making selenate (SeO2−
4) instead. They are strong oxidizing agents.
Alum crystal with small amount of chrome alum to give a slight violet
In some cases, solid solutions of alums with different monovalent and
trivalent cations may occur.
In addition to the alums, which are dodecahydrates, double sulfates
and selenates of univalent and trivalent cations occur with other
degrees of hydration. These materials may also be referred to as
alums, including the undecahydrates such as mendozite and kalinite,
hexahydrates such as guanidinium (CH
3) and dimethylammonium ((CH
2) "alums", tetrahydrates such as goldichite, monohydrates such as
thallium plutonium sulfate and anhydrous alums (yavapaiites). These
classes include differing, but overlapping, combinations of ions.
Other double sulfates
A pseudo alum is a double sulfate of the typical formula ASO
2O, where A is a divalent metal ion, such as cobalt (wupatkiite),
manganese (apjohnite), magnesium (pickingerite) or iron (halotrichite
or feather alum), and B is a trivalent metal ion.
Double sulfates with the general formula A
2O are also known, where A is a monovalent cation such as sodium,
potassium, rubidium, caesium, or thallium(I), or a compound cation
such as ammonium (NH+
4), methylammonium (CH
3), hydroxylammonium (HONH+
3) or hydrazinium (N
5), B is a trivalent metal ion, such as aluminium, chromium, titanium,
manganese, vanadium, iron(III), cobalt(III), gallium, molybdenum,
indium, ruthenium, rhodium, or iridium. Analogous selenates also
occur. The possible combinations of univalent cation, trivalent
cation, and anion depends on the sizes of the ions.
A Tutton salt is a double sulfate of the typical formula A
2O, where A is a univalent cation, and B a divalent metal ion.
Double sulfates of the composition A
4, where A is a univalent cation and B is a divalent metal ion are
referred to as langbeinites, after the prototypical potassium
List of minerals
Gum bichromate photo prints and other similar processes use alums,
sometimes as colloid (gelatin, albumen) hardeners
^ a b Austin, George T. (1984). Shreve's Chemical process industries
(5th ed.). New York: McGraw-Hill. p. 357.
^ http://www.invivogen.com/alhydrogel. Missing or empty title=
^ a b c d Chisholm 1911, p. 766.
^ a b Chisholm 1911, p. 767.
^ a b c Alumen, and the Several Varieties of it; Thirty-eight
Remedies., Pliny the Elder, The Natural History, book 35, chapter 52;
on the Perseus Digital Library at Tufts University. Last accessed 27
^ Dioscorides, book 5, chapter 123.
^ Chisholm 1911, pp. 766-767.
^ A. Archontidou 2005, "Un atelier de preparation de l'alun a partir
de l'alunite dans l'isle de Lesbos" in L'alun de Mediterranée. ed P.
Borgard et al.
^ A. J. Hall & E. Photos-Jones "The nature of Melian alumen and
its potential for exploitation in Antiquity" in Bogard
^ M.Picon et al. 2005, "L'alun des oasis occidentales d'Egypte:
researches sur terrain et recherches en laboratoire" in Bogard
^ George Ernst Stahl (1703), Specimen Beccherianum. Johann Ludwig
Gleditsch, Leipzig. From p. 269: "CVII. Vitriolum, Creta præcipitari
potest, ut omissa metallica sua substantia, aluminosum evadat." (107.
Sulfuric acid [and] chalk can [form a] precipitate, as its liberated
metallic substance, alum, escapes.)
^ George Ernst Stahl (1723), Ausführliche Betrachtung und
zulänglicher Beweiss von den Saltzen, daß diesselbe aus einer zarten
Erde, mit Wasser innig verbunden, bestehen (Detailed treatment and
adequate proof of salts, that they consist of a subtile earth
intimately bound with water) Wäysenhaus, Halle From p. 305: " … wie
aus Kreide und Vitriole-Spiritu, ein rechter Alaun erwächset: … " (
… as from chalk and sulfuric acid, a real alum arises: … )
^ Johann Heinrich Pott (1746), Chymische Untersuchungen, welche
fürnehmlich von der Lithogeognosia oder Erkäntniß und Bearbeitung
der gemeinen einfacheren Steine und Erden ingleichen von Feuer und
Licht handeln [Chemical investigations which primarily concern
lithogeognosia or knowledge and processing of common simple rocks and
earths as well as fire and light]. Potsdam, (Germany), Christian
Friedrich Voss, volume 1, p. 32. From p. 32:] "Concentrirt man
hingegen diese solution gelinde, und läßt sie crystallisiren, so
schiessen harte und mercklich adstringente und hinter her etwas
süßliche crystallen an, die allen Umständen nach in der Haupt-Sach
nichts anders sind als ein formaler Alaun. Diese Entdeckung ist in der
physicalischen Chymie von Wichtigkeit. Man hat bishero geglaubt, die
Grund-Erde des Alauns sey eine in acido Vitrioli solvirte kalckige …
Erde, … " (On the other hand, if one gently concentrates this
solution, and lets it crystallize, then there precipitate hard,
noticeably astringent crystals with a somewhat sweet aftertaste, which
in all circumstances are mainly nothing other than a form of alum.
This discovery is of importance to chemistry. One had hitherto
believed [that] the fundamental earth of alum is a calcareous …
earth dissolved in sulfuric acid, … )
Andreas Sigismund Marggraf
Andreas Sigismund Marggraf (1754), "Expériences faites sur la terre
d'alun" (Experiments made on the earth of alum), Mémoires de
l'Académie des sciences et belles-lettres de Berlin, pp. 41-66.
^ Marggraf (1754) "Expériences qui concernent la régénération de
l'alun de sa propre terre, l'après avoir séparé par l'acide
vitriolique ; avec quelques compositions artificielles de l'alun
par moyen d'autres terres, et dudit acide" (Experiments that concern
the regeneration of alum from its own earth, after having separating
it by sulfuric acid ; with some artificial compounds of alum by
means of other earths and the aforesaid acid), Mémoires de
l'Académie des sciences et belles-lettres de Berlin, pp. 31-40.
Torbern Bergman (1767), "IX. De confectione Aluminis". In Opuscula
physica et chemica, I. G. Müller, Leipzig, 1788), volume 1. On pp.
306-307, after noting that Marggraf had noticed that potash caused
alum to crystallize from a solution of alumina and sulfuric acid,
Bergman adds "Notatu quoque dignum est, quod hoc cristallisationis
obstaculum alcali volatili aeque tollatur, non vero alkali minerali et
calce." (It is significant as well that by [use of] the volatile
alkali [i.e., ammonia] this obstacle to crystallization is similarly
removed, but not [in the cases of] mineral alkali [i.e., sodium
carbonate] and lime.)
Martin Heinrich Klaproth
Martin Heinrich Klaproth (1797), Beiträge zur Chemischen Kenntniss
Der Mineralkörper (Contributions to [our] chemical knowledge of
mineral substances). Decker and Co., Posen, and Heinrich August
Rottmann, Berlin; pp. 45-46 and p. 193.
Martin Heinrich Klaproth
Martin Heinrich Klaproth (1801), Analytical Essays Towards Promoting
the Chemical Knowledge of Mineral Substances. T. Cadell, Jr. & W.
Davies, London. His finding of potassium in leucite appears on pp.
353-354.: "On the contrary, I was surprised in an unexpected manner,
by discovering in it another constituent part, consisting of a
substance, the existence of which, certainly, no one person would have
conjectured within the limits of the mineral kingdom … This
constituent part of leucite … is no other than pot-ash, which,
hitherto, has been thought exclusively to belong to the vegetable
kingdom, and has, on this account, been called VEGETABLE ALKALI. —
This discovery, which I think of great importance, cannot fail to
occasion considerable changes in the systems of natural history, …
.". The discovery of potassium in lepidolite is mentioned on p. 472.
^ Vauquelin (1797) "Sur la nature de l'Alun du commerce, sur
l'existence de la potasse dans ce sel, et sur diverses combinaisons
simples ou triples de l'alumine avec l'acide sulfurique". In Annales
de Chimie et de Physique, 1st series, volume 22, pages 258-279.
Jean-Antoine Chaptal (1797), "Comparée des quatre principales
sortes d'Alun connues dans le commerce; et Observations sur leur
nature et leur usage". In Annales de Chimie et de Physique, 1st
series, volume 22, pages 280-296.
^ Otto Helmboldt, L. Keith Hudson, Chanakya Misra, Karl Wefers,
Wolfgang Heck, Hans Stark, Max Danner, Norbert Rösch "Aluminum
Compounds, Inorganic" in Ullmann's Encyclopedia of Industrial
Chemistry 2007, Wiley-VCH, Weinheim.doi:10.1002/14356007.a01_527.pub2
^ J. Locke (1902). "On some double suphates of thallic thallium and
caesium". American Chemical Journal. 27: 281.
^ Bell, Chichester H. (1887). Summarizing original article by C. Fabre
(Compt. rend., 105, 114–115). "
Selenium Alums". Abstracts of
chemical papers. Inorganic chemistry. Journal of the Chemical Society.
Volume LII. Part II.: 1014. Retrieved 2017-08-19.
Halotrichite on Mindat.org
^ Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the
Elements (2nd Edn.), Oxford: Butterworth-Heinemann.
This article incorporates text from a publication now in
the public domain: Chisholm, Hugh, ed. (1911). "Alum".
Encyclopædia Britannica. 1 (11th ed.). Cambridge University Press.
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