Published: 2015-12-25 Views: 1416
HOEV: OPNAME VAN BESIENSWAARDIGHEDE Nr 026
HEHA: SURVEY OF HERITAGE SITES No 026
KALKOONDE OP KALKHEUWEL
GPS data:X 27.52.17 Y –25.48.58
PAD: K 27 (Lanseriapad)
Kalk was in die negentiende eeu vir blankes die belangrikste bindmiddel in bouwerk in Suid-Afrika voordat sement hier vervaaardig is. Kalk is ook vir die afwit van mure gebruik en as ontsmettingsmiddel in landbou en elders. Dit word ook gebruik om die ph- waarde van landbougrond te reguleer, asook in die vervaardiging van yster en staal en klein hoeveelhede is gebruik om vuurwerke (helder wit kalklig) te maak.
Kalk kom in die natuur in die vorm van CaCO3 (kalsiumkarbonaat of kalkklip) voor. Dolomiet is so ‘n kalkklip en die rante en berge suid van die Witwatersberg bestaan hoofsaaklik hieruit.
Wanneer hierdie kalkklip of kalsiumbarbonaat met hout- of koolvure “gebrand” word, verander dit na CaCo (kalsiumoksied of “gebrande kalk”). Dit is ‘n blou-wit poeier wat met water verbind om Ca(OH)3 (kalsiumhidroksied of “bebluste kalk”) te vorm. Die proses gee baie warmte af. Die gebrande kalk is ook baie vretend en is daarom dikwels aangewend om besmette karkasse te vernietig.
Gebluste kalk word met water en sand gemeng en as mortel of messelklei in bouwerk gebruik. Mettertyd verbind die gebluste kalk met koolsuurgas in die lug en vorm weer kalsiumkarbonaat wat ‘n kristallyne struktuur het en die sandkorrels in die messelmortel “kliphard” aan mekaar bind. Om die proses te verhaas is die hoop mortel nat gehou en vir ‘n paar dae “ingesuur” terwyl dit omgeroer word sodat alle dele met die koolsuurgas van die lug in aanraking kom. Soms is daar ook vuur in en om nuutgeboude geboue gemaak sodat die koolsuurgas van die rook die kalk verhard.
Na die ontdekking van goud het die ZAR ‘n ongekende ekonomiese opbloei beleef wat met grootskaalse boubedrywighede gepaard gegaan het. Hiervoor moes daar baie kalk ontgin word. Aangesien Kalkheuwel, wat sy naam ook daardeur gekry het, naby die stede was, is daar baie kalkoonde in die gebied bedryf. Hierdie oonde bestaan uit ronde gemesselde silinders van 3m deursnee en 6m tot 9m hoogte wat gewoonlik teen ‘n grondwal of bank gebou is sodat die kalkklip maklik bo ingegooi en die gebrande kalk onder uitgekrap kon word. Met die koms van sementfabrieke is kalk baie minder gebruik en het die kalkoonde in onbruik geraak. Baie van hulle staan vandag nog as herinnering aan die vorige eeue se bedrywighede.
Die dolomiet bevat ook baie fossiele van diere en mense wat meer as twee miljoen jaar gelede hier geleef het. Die voorkoms van hierdie fossiele gebeendere in die dolomiet is toevallig by kalkoonde ontdek. Die hele gebied rondom en ook insluitend Kalkheuwel is in 2001 as Wêrelderfenisterrein verklaar waardeur die ontginning van kalk belet is. Nou sal die kalkoonde die fossiele van die toekoms wees.
Een van die ou kalkoonde van bo af gesien. Let op die merke wat die vuur gemaak het.
As ‘n mens hier in die nag sou loop, kon jy nogal in so ‘n oond val.
Dikwels is twee of meer kalkoonde langs mekaar gebou.
Two or more lime kilns were often built in a row where a suitable embankment was available.
LIME KILNS OF KALKHEUWEL
GPS data: X 27.52.17 Y –25.48.58
ROAD K 27 (Lanseria road)
Neither the Stone Age nor the Iron Age people made use of lime. But white pioneers came to these regions with the tradition of using lime as binding material in building mortar and plaster or for lime washing of plastered walls. It was also and is still used in agriculture to regulate the ph value of soils and to discard of infected carcasses. On a small scale was also used in fire works and stage lighting in the form of bright white “lime light”.
In nature we find lime in the form of CaCo3 (calcium carbonate or limestone). Dolomite, of which the mountains to the south of the Witwatersberg are composed, is mainly a limestone.
When limestone is fired with wood or charcoal fires at a temperature of 1200-1400 degrees C, it turns into CaO3 (calcium oxide or burnt / unslaked lime, also called “quicklime” for its corrosive action). This is a blue-white powder, which reacts with water to form Ca (HO) 3 (calcium hydrate, called hydrated or slaked lime). This process relieves intensive heat.
The slaked lime is mixed with sand and water and used as building mortar. Over time the calcium hydrate reacts with carbon dioxide in the air and turns into calcium carbonate again which is a hard crystalline material that binds the sand grains together. To hasten up the process, the mixed heap of mortal was kept wet and turned around for some days to expose it to carbon dioxide and some times fires were made in and around newly built houses. The same process takes place in lime wash, only that no sand is used, but pigments of coloured ground or clay were used in the old times.
After the discovery of gold, the ZAR (Zuid-Afrikaansche Republiek) went into a period of rapid development with a building boom that used up a lot of lime. With its convenient nearby position, Kalkheuwel (Afrikaans word for “Lime Hill”) was then extensively mined for lime. The dolomite was excavated and burnt in the numerous limekilns that were operated on Kalkheuwel. These were 3m diameter and 6m to 9m cylinders built of brick into embankments so as to make easy to feed the wood and limestone from the top and to remove the burnt lime at the bottom. When cement was made in South Africa in the beginning of the twentieth century, lime got out of common use. The limekilns at Kalkheuwel were left behind and today stand there as monumental reminders of yesteryears’ activities.
The dolomite also contains millions of fossil bones of animals and humans that lived in the area more than two million years ago. The first of these fossils was discovered at a working limekiln. In 2001 the whole area around and including Kalkheuwel was declared a World Heritage Site so that dolomite mining is forbidden in future. Now the limekilns are the “to be protected” heritage relicts.