Waterlogged lignite

Further to my posting about the conservation of lignite in
Conservation DistList Instance: 18:17 Thursday, October 7, 2004, I
would like to add a note of explanation.

I have found that there is a problem of terminology when people talk
about lignite--strictly speaking, the term should be restricted to a
kind of fossilised wood, halfway between peat and coal.  There is a
lot of this mined in North Dakota and eastern Germany, where it is
known as Brown Coal or Braunkohle.

The objects I was talking about are actually made of Kimmeridge
shale, which is found on the south coast of England, but was
exported all over the Roman Empire.  This is more like slate
impregnated with crude oil--in fact, there are a few oil wells in
the Kimmeridge area.  Kimmeridge shale is black and shiny and can be
easily turned to make bangles, beads etc.  I don't think you could
actually work lignite--it is too soft and tends to crumble as it
dries out.  After excavation, shale objects tend to dry out and
laminate, and the colour becomes dull and grey.  The PEG treatment
consolidates them and helps to preserve the colour.  I also omitted
to mention that this treatment was developed by Andrew Oddy and
Hannah Lane at the British Museum, and was written up by them in
Studies in Conservation 21 (1976) 63-66.

Both these substances are different from jet, which is also related
to coal. It is black and shiny but breaks with a conchoidal
fracture.  Jet is also found in England and is particularly
associated with Whitby, in Yorkshire. It was much used in the Roman
period for making beads etc.  Jet tends to be well preserved and
does not usually require any treatment after excavation. The major
difference between lignite and jet on the one hand, and shale on the
other, is that lignite and jet are mostly organic with a small
amount of mineral matter, while shale is mostly inorganic with a
small amount of organic matter.

Dr Barry Knight
Head of Conservation Research
The British Library
96 Euston Road
London NW1 2DB
+20 7412 7229
Fax: +44 20 7412 7658


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                  Conservation DistList Instance 18:20
                Distributed: Thursday, November 4, 2004
                       Message Id: cdl-18-20-001
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