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29. Greywacke

Based on 'Rock glazes of NZ' Minna Bondy

Sample 12. Semi Schistose argillite or fine-grained greywacke

Sample 13. Greywacke

Sample 41. Greywacke

This is a hard dark blue-grey rock-a strongly indurated mudstone. It shows a tendency to split into irregular sub-parallel layers. The argillite is widespread throughout New Zealand and commonly interbedded with greywacke.

A very hard even-grained sandstone, dark grey, green-grey, or blue in colour. Composition of these rocks is variable. Typical N.Z. examples contain about 70% Silica and 12%-14% Alumina.

Greywacke was often used in New Zealand for glazing to produce a Tenmoku like finish.

I chose to test a sample sourced from a Greywacke/Argillite quarry near Morrinsville in the North Island. The reason for this was that there were large piles of fine powder under the crusher, which saved a lot of processing time. It also meant that the samples from the quarries could be obtained in larger quantities if I wanted to source more.

Bondy tested several greywacke samples from around the Wellington area. The first sample I sourced was from the Remutaka Hill road.

Both sedimentary rocks originally, greywacke was formed from sandy sediment whereas argillite would have been a finer, silty mud or clay.

JC. Schofield includes a greywacke glaze in his book "Materials for the New Zealand Potter" 1970. Schofield's book has many glaze recipes with no images of results, I hope to map and document these sites next.

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