Based on 'Rock glazes of NZ' Minna Bondy
Sample 22.
Characteristics " frothy " appearance. Identical with ordinary household pumice. Good Specimens may be collected from the beaches around Lake Taupō.
Similar in composition to Rhyolite and Obsidian
Pumice, Obsidian, Rhyolite and Ignimbrite are all similar in composition and appear commonly in the central volcanic district of the North Island of New Zealand. This sample was sourced from Castlecliff Beach, Whanganui, having travelled down the Whanganui river from the central North Island, out to sea then washing back up on the tide.
These igneous rocks, formed from magma, are characterised by how the molten material cools after volcanic activity. I was told to think about Pumice and Obsidian like a glass of beer. As the beer is poured from the tap the clear glassy liquid settles to the bottom, whereas the gassy aerated beer forms a head on top. Pumice is much the same, having cooled from a molten state rapidly in water. The gases forming air pockets as they escape resulting in a bubbly aerated rock.
This means the pumice and obsidian are going to produce similar glazes, and due to the high silica and feldspars, they produce beautiful silky celadons. Pumice is easy to crush and grinds well, though a decent quantity is needed, as it really is mostly air.