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.