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Porcellanite

Material name: Porcellanite
Synonyms: Porcelain jasper
Material (geologic): Porcellanite

Detail of sample of porcellanite
Foto: Rengert Elburg, 1999

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General characteristics

(References in one of the next updates)

Material: Porcellanite is, like the name already suggests a material very similar to porcelain. Basically it is a natural ceramic formed by thermal metamorphism of, mostly kaolinitic, clays. This thermal metamorphism can either be induced by spontaneous combustion of lignite (brown coal) or, as is mostly the case, by volcanic activity.
Some caution has to be taken when using the term Porcellanite, as the definition is very unclear. In some texts it is used for very impure chert or indurated clay or shale and in the Hungarian literature it has been used to describe the parent rock of radiolarite, like at Szentgál-Tüzköveshegy. We clearly prefer to restrict its use to the material described on this page, and materials that were formed under similar conditions.
Colour: In a freshly flaked state mostly grey to bluish, but porcellanite is very susceptible to weathering. It the becomes mostly white, grayish or like in the picture above reddish. Porcellanite we found in a Final Paleolithic context had become positively chalk-like with a pinkish colour.
Other information: In Central Europe most porcellanites are formed by Tertiary volcanism, and the occurrences of this material are therefore closely linked with volcanic sediments from that age.
Knapping notes: Easily knappable, reasonable straight fracture, edges are not very sharp. All in all very mediocre.
Archaeological description: In Central Europe, porcellanite is mostly used as a local substitute if high quality materials are unavailable, but the above mentioned Late Palaeolithic material, must have been transported over at least 50 kilometers. In other regions, e.g. Ireland, Porcellanite is used for axes and was 'traded' extensively.

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Korozluky-Špičák

Locality: Korozluky-Špičák, Most county, Northern Bohemia, Czech Republic
Synonyms: N/A
Geographic description: Korozluky lies approx. 2 kilometers to the west from the sources of the Skršin-type quartzite on the most western fringes of the České Středohoří (Czech midmountains) Nature Conservation Area approx. 7 km east of Most in Northern Bohemia. Špičák is the toponym of the volcanic hill directly to the north of the sampling site.
Geographical co-ordinates: Lat. 50 28' 43" N
Long. 013 45' 04" E
(Mapdatum WGS 84)

Co-ordinate precision: Taken with a hand held GPS-receiver, within approx. 20 meters of the sampling spot, but exact primary outcrop is (still) unknown.
Other topographical information: The only way to reach the sampling site is by foot or with a very good four-wheel drive. The very bumpy track leads from Korozluky to Dobrčice. The sample was taken quite exactly between the two villages.
Additional information: Volcanic exposure near sampling site
Foto: Rengert Elburg, 1999
  As the sampling site itself is not very photogenic, we publish this picture of a very nice volcanic exposure at Špičák, approx. 200 meters from the site where we sampled the porcellanite.
Visitors information: Next place for a beer (or a coffee) is Skršin.
Sampling information: The sample shown was found along the bank of a track, but comes very likely from a locality in the immediate vicinity.
For a full-blown picture of sample,
click here (88 KBytes).
block of porcellanite from Northern Bohemia
Foto: Rengert Elburg, 1999
Thin section under plane polarized light
Foto: Marlina Elburg, 2000
For a full-blown picture of thin section,
click here (81 KBytes).
Sample description: To get a better idea of how this material might form and from what kind of substrate, we had a piece made into the thin section you can see above. Alas, the material is completely opaque except for small fiamme of cryptocrystalline quartz, as our geologist tells us. Under cross polarized light, even less was visible. We still assume that we have naturally baked clay or something very similar on our hands here, but with the thin section we can't prove it.
 

Last modified
January 6, 2002
Contents primarily by:
Rengert Elburg
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