Editing Limescale

{{Short description|Hard, chalky deposit of calcium carbonate}}
[[File:Limescale-in-pipe.jpg|thumb|Limescale build-up inside a pipe reduces both liquid flow through the pipe and thermal conduction from the liquid to the outer pipe shell. Both effects will reduce the pipe's overall thermal efficiency when used as a [[heat exchanger]].]]

'''Limescale''' is a hard, [[chalk]]y deposit, consisting mainly of [[calcium carbonate]] (CaCO<sub>3</sub>). It often builds up inside [[kettle]]s, [[boiler]]s, and pipework, especially that for hot water. It is also often found as a similar deposit on the inner surfaces of old pipes and other surfaces where [[hard water]] has flowed. Limescale also forms as [[travertine]] or [[tufa]] in hard water springs.

The colour varies from off-white through a range of greys and pink or reddish browns, depending on the other minerals present. Iron compounds give the reddish-browns.  

In addition to being unsightly and hard to clean, limescale can seriously damage or impair the operation of various plumbing and heating components.<ref name=Ullmann>[[Hermann Weingärtner]], "Water" in ''[[Ullmann's Encyclopedia of Industrial Chemistry]]'', December 2006, Wiley–VCH, Weinheim. {{doi|10.1002/14356007.a28_001}}</ref> [[Descaling agent]]s are commonly used to remove limescale. Prevention of [[fouling]] by scale build-up relies on the technologies of [[water softening]] or other water treatment.

Limescale can also affect optic products, such as [[glasses]] and [[mirror]]s.<ref> https://phys.org/news/2022-09-method-limescale.pdf</ref>
[[File:Eifelmarmor01.jpg|thumb|upright|This column in the [[Bad Münstereifel]] church in Germany is made from the calcium carbonate deposits that built up in the Roman [[Eifel Aqueduct]] over several centuries of use.]]

== Chemical composition ==
The type found deposited on the [[heating element]]s of [[water heating|water heaters]] consists mainly of [[calcium carbonate]] (CaCO<sub>3</sub>). Hard water contains [[calcium]] (and often [[magnesium]]) [[bicarbonate]] or similar ions. Calcium, magnesium, and carbonate ions dissolve from rocks through which rainwater percolates before collection. Calcium salts, such as [[calcium carbonate]]{{citation needed|reason=the [[calcite]] article says "Calcite exhibits an unusual characteristic called retrograde solubility: it is less soluble in water as the temperature increases." This article appears to contradict it|date=February 2024}}  and [[calcium bicarbonate]] (Ca(HCO<sub>3</sub>)<sub>2</sub>), are more [[solubility|soluble]] in hot water than cold water; thus, heating water alone does not cause calcium carbonate to precipitate. However, there is an [[chemical equilibrium|equilibrium]] between dissolved calcium bicarbonate and dissolved calcium carbonate as represented by the chemical equation

:{{chem2|Ca(2+) + 2 HCO3(-) <-> Ca(2+) + CO3(2-) + CO2 + H2O}}

Note that [[carbon dioxide|CO<sub>2</sub>]] is dissolved in the water. Carbon dioxide dissolved in water (aq) tends to equilibrate with carbon dioxide in the gaseous state (g):

:{{chem2|CO2 (aq) <-> CO2 (g)}}

The equilibrium of CO<sub>2</sub> moves to the right, toward gaseous CO<sub>2</sub>, when water temperature rises or pressure falls. When water that contains dissolved calcium carbonate is warmed, CO<sub>2</sub> leaves the water as gas, this reduces the amount involved in the reaction causing the equilibrium of bicarbonate and carbonate to re-balance to the right, increasing the concentration of dissolved carbonate. As the concentration of carbonate increases, calcium carbonate precipitates as the [[salt (chemistry)|salt]]: 

:{{chem2|Ca(2+) + CO3(2-) -> CaCO3}}

In pipes as limescale and in surface deposits of calcite as travertine or tufa the primary driver of calcite formation is the exsolution of gas. When heating hard water on the stove, these gas bubbles form on the surface of the pan prior to boiling. Gas [[Solid solution|exsolution]] can also occur when the confining pressure is released such as removing the top off a beer bottle or where subsurface water is flowed into an atmospheric pressure tank.

As new cold water with dissolved calcium carbonate/bicarbonate is added and heated, the process continues: CO<sub>2</sub> gas is again removed, carbonate concentration increases, and more calcium carbonate precipitates.

Scale is often colored because of the presence of [[iron]]-containing compounds. The three main iron compounds are [[wüstite]] (FeO), [[hematite]] ({{chem2|Fe2O3}}), and [[magnetite]] ({{chem2|Fe3O4}}).

==As a stone==
The Roman [[Eifel Aqueduct]] was completed around 80 AD and broken and largely destroyed by Germanic tribes in 260. By the Middle Ages the limestone-like limescale accretions from the inside of the aqueduct were particularly desirable as a building material, called "Eifel marble" in an area with little natural stone. In the course of operation of the aqueduct, many sections had a layer as thick as {{convert|20|cm|in|0}}. The material had a consistency similar to brown [[marble]] and was easily removable from the aqueduct. Upon polishing, it showed veins, and it could also be used like a stone board when cut flat. This artificial stone found use throughout the Rhineland and was very popular for [[column]]s, window frames, and even [[altar]]s. Use of "Eifel marble" can be seen as far east as [[Paderborn]] and [[Hildesheim]], where it was used in the [[cathedral]]s. [[Roskilde Cathedral]] in [[Denmark]] is the northernmost location of its use, where several gravestones are made of it.<ref>{{cite book |last1=Tegethoff |first1=F. Wolfgang|last2=Rohleder |first2=Johannes |last3=Kroker |first3=Evelyn |url=https://books.google.com/books?id=pbkKGa19k5QC&dq=%22eifel+aqueduct%22&pg=PA93 |title=Calcium Carbonate: From the Cretaceous Period Into the 21st Century |work=Birkhäuser |year=2001 |publisher=Springer |isbn=3-7643-6425-4}}</ref>

Trade to the west took it to England as a high-status export material in the 11th and 12th centuries, where it was made into columns for a number of [[Norman architecture|Norman]] English Cathedrals. The impressive polished brown stone was known for many years as 'Onyx Marble'. Its origin and nature was a mystery to people studying the stonework at [[Canterbury Cathedral]], until its source was identified in 2011.<ref>{{cite book| author=C. Wilson|year=2015| chapter=Canterbury Cathedral's Mystery 'Marble': A Double Imposture Unmasked' |pages=156–60| editor= P. Fergusson| title=Canterbury Cathedral Priory in the Age of Becket |location=New Haven and London |isbn=9780300175691 }}</ref> It is used there as columns supporting the cloister roof, alternating with columns of Purbeck Marble. These large cathedral cloisters needed several hundred such columns around an open quadrangle, which must have been supplied by a well-organized extraction and transport operation. The Eifel deposits, now called [[Calcareous sinter]] or calc-sinter (since it is neither [[onyx]] nor [[marble]]), have also been identified at [[Rochester Cathedral|Rochester]]<ref>{{cite journal| author=John McNeill| title=The Romanesque Cloister in England| journal=Journal of the British Archaeological Association| volume=168 |year=2015|pages=34–76| doi=10.1179/0068128815Z.00000000038| s2cid=194154048| url=https://www.academia.edu/19984110}}</ref> and in the now lost [[Norman architecture|Romanesque]] cloister at [[Norwich Cathedral|Norwich]]<ref>{{cite journal|author= R.B. Harris| year=2019| title=Reconstructing the romanesque cloister of Norwich cathedral | journal=The Antiquaries Journal| publisher=Cambridge University Press| volume=99|pages=133–159| doi=10.1017/S0003581519000118| s2cid=203298501}}</ref> as well as the Infirmary Cloisters, Chapter House windows, and Treasury doorway at Canterbury.<ref>{{cite web|url=http://www.canterbury-archaeology.org.uk/stones-calc/4594124807| title=calc-sinter or Onyx Marble| author=Geoff Downer|year=2019|website=canterbury-archaeology.org.uk| publisher=Canterbury Historical & Archaeological Society (CHAS)}}</ref>

==Related material==
[[Soap scum]] forms when calcium [[cation]]s from hard water combine with [[soap]], which would dissolve in soft water. This precipitates out in a thin film on the interior surfaces of baths, sinks, and drainage pipes.
==Gallery==
<gallery mode="packed" heights="180px">
File:Limescale from the Kettle.jpg|Several large pieces of limescale removed from an electric [[kettle]].
File:Kesselstein k.jpg|A [[scanning electron microscope|scanning electron ]] [[micrograph]] of limescale (The [[field of view]] is 64 × 90 μm).
</gallery>

==See also==

*[[Fouling]]
*[[Hard water]]
*[[Limestone]]
*[[Water softening]]

==References==
{{reflist}}

[[Category:Fouling]]
[[Category:Limestone]]
[[Category:Petrology]]
[[Category:Water]]