Editing Alum (section)

==History==

===Alum found at archaeological sites===
The western desert of Egypt was a major source of alum substitutes in antiquity. These [[evaporite]]s were mainly {{chem|Fe|Al|2|(SO|4|)|4|·22 H|2|O}}, {{chem|Mg|Al|2|(SO|4|)|4|·22 H|2|O}}, {{chem|NaAl(SO|4|)|2|·6 H|2|O}}, {{chem|Mg|SO|4|·7H|2|O}} and {{chem|Al|2|(SO|4|)|3|·17 H|2|O}}.{{refn|name=Picon-etal-2005-lAlun|
{{cite conference
 |author1=Picon, M.
 |author-link=Maurice Picon
 |display-authors=etal
 |date=2005
 |title=L'alun des oasis occidentales d'Egypte: Researches sur terrain et recherches en laboratoire}} 
}} <ref name="Borgard-Brun-Picon-2005-06-07-alunMedit" />
The [[Ancient Greece|Ancient Greek]] [[Herodotus]] mentions Egyptian alum as a valuable commodity in ''The Histories''.<ref name=Herodotus-c430ʙᴄᴇ-Hists/>

The production of potassium alum from [[alunite]] is archaeologically attested on the island [[Lesbos]].{{refn|name=Archontidou-2005-alunMedit|
{{cite conference
 |last=Archontidou |first=A.
 |date=2005
 |title=Un atelier de preparation de l'alun a partir de l'alunite dans l'isle de Lesbos
 |language=fr,grc
}}
<ref name="Borgard-Brun-Picon-2005-06-07-alunMedit" />
}}
The site was abandoned during the 7th&nbsp;century&nbsp;CE, but dates back at least to the 2nd&nbsp;century&nbsp;CE. Native ''alumen'' from the island of [[Melos]] appears to have been a mixture mainly of alunogen ({{chem|Al|2|(SO|4|)|3|·17 H|2|O}}) with potassium alum and other minor sulfates.{{refn|name=Hall-Jones-2005-alunMedit|
{{cite conference
 |first1=A. J. |last1=Hall 
 |first2=E. |last2=Photos-Jones
 |date=2005
 |title=The nature of Melian alumen and its potential for exploitation in Antiquity
}}
<ref name="Borgard-Brun-Picon-2005-06-07-alunMedit" />
}}

===Alumen in Pliny and Dioscorides===
A detailed description of a substance termed ''alumen'' occurs in the [[Roman Empire|Roman]] [[Pliny the Elder]]'s ''[[Natural History (Pliny)|Natural History]]''.<ref name="PlinySr-NatHist" />

By comparing Pliny's description with the account of ''stypteria'' (στυπτηρία) given by [[Dioscorides]],<ref name="Dioscor-MatMed" /> it is obvious the two are identical. Pliny informs us that a form of ''alumen'' was found naturally in the earth, and terms it ''salsugoterrae''.

Pliny wrote that different substances were distinguished by the name of ''alumen'', but they were all characterised by a certain degree of [[Astringent|astringency]], and were all employed for dyeing and medicine. Pliny wrote that there is another kind of alum that the ancient [[Ancient Greece|Greeks]] term ''schiston'', and which "splits into filaments of a whitish colour".<ref name=PlinySr-NatHist/> From the name ''schiston'' and the mode of formation, it seems that this kind was the salt that forms spontaneously on certain salty minerals, as alum [[slate]] and bituminous [[shale]], and consists mainly of sulfates of iron and aluminium.{{citation needed|date=May 2009}} One kind 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 (chemistry)|solution]] of [[Iron(II) sulfate|iron{{sup|II}} 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 types 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.<ref name=Chisholm-1911/>{{rp|pages=&nbsp;766–767}}

===Alum described in medieval texts===
Alum and [[Iron(II) sulfate|green vitriol]] (iron sulfate) both have sweetish and astringent taste, and they had overlapping uses. Therefore, through the [[Middle Ages]], alchemists and other writers do not seem to have distinguished the two salts accurately. In the writings of the [[Alchemy|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 indiscriminately to both.{{citation needed|date=June 2023}}

Alum was the most common [[mordant]] (substance used to set dyes on fabrics) used by the dye industry, especially in Islamic countries, during the [[Islamic Golden Age|middle ages]]. It was the main export of the [[Chad]] region, from where it was transported to the markets of [[Egypt]] and [[Morocco]], and then to [[Europe]].  Less significant sources were found in Egypt and [[Yemen]].<ref name=Goitein-Sanders-1967/>

===Modern understanding of the alums===
During the early 1700s, [[Georg Ernst Stahl|G. E. Stahl]] claimed that reacting sulfuric acid with limestone produced a sort of alum.<ref name=Stahl-1703/>{{efn|
CVII.  ''Vitriolum, ''Creta'' præcipitari potest, ut omissa metallica sua substantia, ''aluminosum'' evadat.''
:
:[107.  Sulfuric acid {{grey|[and]}} ''chalk'' can {{grey|[form a]}} precipitate, as its liberated metallic substance, ''alum'', escapes.]
:
''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]
:::— G. E. Stahl (1703)<ref name=Stahl-1703/>}}{{efn|Wäysenhaus, [[Halle (region)|Halle]]
''...&nbsp;wie aus Kreide und Vitriole-Spiritu, ein rechter Alaun erwächset:&nbsp;...''
:
:[...&nbsp;as from chalk and sulfuric acid, a real alum arises&nbsp;...]
:::— G. E. Stahl (1723)<ref name=Stahl-1723/>}}<ref name=Stahl-1723/> The error was soon corrected by [[Johann Heinrich Pott]] and [[Andreas Sigismund Marggraf]], who showed that the [[Precipitation (chemistry)|precipitate]] obtained when an alkali is poured into a solution of alum, namely [[alumina]], is quite different from [[Lime (mineral)|lime]] and [[chalk]], and is one of the ingredients in common [[clay]].<ref name=Pott-1746-Chym-Untrs/>{{efn|''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,&nbsp;...''
:
:[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 {{grey|[that]}} the fundamental earth of alum is a calcareous ... earth dissolved in sulfuric {{nobr|acid,&nbsp;...]}}
::: — J. H. Pott (1746)<ref name=Pott-1746-Chym-Untrs/>
}}<ref name=Marggraf-1754/>{{rp|pages=41–66}}

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.<ref name=Chisholm-1911/>{{rp|page=766}}<ref name=Marggraf-1754/>{{rp|page=31–40}} In 1767, [[Torbern Bergman]] observed the need for potassium or ammonium sulfates to convert [[aluminium sulfate]] into alum, while sodium or calcium would not work.<ref name=Bergman-1767/>{{efn|
After acknowledging 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 {{grey|[use of]}} the volatile alkali (''i.e., ammonia'') this obstacle to crystallization is similarly removed, but not {{grey|[in the cases of]}} mineral alkali]
::(''i.e., sodium carbonate and lime'').
:::— Bergman (1767)<ref name=Bergman-1767/>}}<ref name=Chisholm-1911/>{{rp|page=766}}

The composition of common alum was determined finally by [[Louis Nicolas Vauquelin|Louis Vauquelin]] in 1797. As soon as [[Martin Heinrich Klaproth|Martin Klaproth]] discovered the presence of potassium in [[leucite]] and [[lepidolite]],<ref name=Klaproth-1797/><ref name=Klaproth-1801/>{{efn|"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 {{nobr|history, ... ." — [[Martin Heinrich Klaproth|M. H. Klaproth]] (1801)<ref name=Klaproth-1801/>}}
}}
[[Louis Nicolas Vauquelin|Vauquelin]] demonstrated that common alum is a [[double salt]], composed of sulfuric acid, alumina, and potash.<ref name=Vauquelin-1797/> In the same journal volume, [[Jean-Antoine Chaptal|Chaptal]] published the analysis of four different kinds of alum, namely, Roman alum, Levant alum, British alum, and an alum manufactured by himself,<ref name=Chaptal-1797/> confirming [[Louis Nicolas Vauquelin|Vauquelin]]'s result.<ref name="Chisholm-1911" />