A Further Investigation of Crozzle

 

A view of crozzle in plane polarised light by Alessandro Da Mommio

At the end of an afternoon spent in Sheffield, on the last day of March in 2021 - when my main objective was to acquire a sample of Silkstone Rock - I posted a selection of some of my more interesting photographs on Facebook. 

Crozzle used for decorative purposes on a boundary wall

 

One of these was a photo of crozzle, the black lava like slag that was produced during the now obsolete process of making steel in a cementation furnace and, when Alessandro Da Mommio said he would love to see it under the microscope, I decided to take another trip to Sheffield and obtain a couple of pieces for him.

Samples of crozzle

When researching definitions of minerals for my Language of Stone Blog, I had often made a link to his Alex Streckeisen website, which I think provide very clear definitions of various rocks and minerals, accompanied by his excellent photomicrographs.

The cementation furnace in Sheffield

The cementation process essentially involved alternate layers of wrought iron and ground charcoal being packed into a stone chest, sealed from the air with wheelswarf and then a fire lit to gradually bring it to red heat - with the load then left at that temperature for 7 to 10 days.

An information panel at the cementation furnace in Sheffield

Having obtained a couple more samples of crozzle from City Road, I selected both black and grey samples and, having carefully packaged them to minimise postage costs and jumped through hoops because of BREXIT, I sent them off to Alessandro in Italy.

 

Samples of crozzle

The black crozzle is black and very dense, often with glassy surfaces, but there are no obvious inclusions of material of a different character. The grey crozzle that I collected, however, is much less dense and contains irregularly shaped inclusions of a soft white material that scratches very easily with a fingernail.

 

A view of crozzle in plane polarized light by Alessandro Da Mommio

I know very little about any of the processes for making steel, or the chemistry of slag, but having made a thin section of the black crozzle and examined it with a petrological microscope, Alex informed me that “it is a bit boring in the sense that it only contains silica, glass and bubbles".

A view of crozzle with crossed polars by Alessandro Da Mommio

Having myself been very interested in examining a wide variety of rocks under a microscope, as a geology undergraduate, and more recently when producing my own Glowing Edges Designs – based on the kaleidoscope of colours very often seen when using crossed polars – I fully appreciate his comments.

 

A view of olivines with crossed polars by Glowing Edges Designs

As seen in nature as quartz, silica appears as a very dull mineral with very little aesthetic merit – viewed with both plane polarised light and crossed polars – and glassy material, for example in volcanic obsidian, virtually disappears from view.

A view of crozzle in plane polarized light by Alessandro Da Mommio

Not having access to a petrological microscope myself and with Alex being very pleased to add such an unusual building material to his collection of thin sections, I consider this to be a great example of post-BREXIT Anglo-Italian collaboration - which will be continued with a piece of coal from Charlton Brook in Sheffield.

A view of crozzle with crossed polars by Alessandro Da Mommio