The workshop is warm, its cave-like ability to stay cool defeated by the insistent heat coming from the kiln in the middle of the room. A line of bright yellow light, which you can feel as much as see, marks out the tight-lipped join between the firebricks of the circular lid and cylinder of its body. Now there is a continuous mains hum as the tightly wound elements embedded in the firebricks work almost continuously to keep the kiln so hot. The kiln has reached temperature for this stoneware firing and will now soak at 1200^oC  for an hour to let the bright cobalt blue and deep metallic iron red of the glazes work their magic as they mingle with each other in their liquid state. It will be another 12 hours before the kiln has cooled enough for me to crack open the kiln in trepidation to see how generous the firing has been to the pots.

I still find it exciting that it is possible to run a device like this which creates the temperatures you would expect to find inside a volcano. Obviously, the scale is rather different, but the ability to melt rocks has an elemental feel to it. There are other connections between igneous rocks and pottery; amongst the glaze ingredients for stoneware are silica (quartz) and potash feldspar both minerals found in granites. Sometimes potters use ground igneous rocks like basalt as a glaze ingredient – a retired potter friend used to use the distinctive red felsite from Harden Quarry near Biddlestone as the secret ingredient in some of her glazes.

The products of a kiln and the rocks which form from magma also share the quality of durability. The ubiquitous find in archaeological digs is ceramic material. Ceramic is not only physically durable, but it is inert to the oxidising conditions that usually exist in the immediate subsurface where air and water can penetrate easily. The content of the ceramic and the way it has been fired also mean that it can be used to give information about its source. In addition, changes in the form, construction and decoration of pots mean that ceramic can be used to help date the stratigraphic layers in which it is found.

The results of igneous activity tend to survive the many possible ways that geological processes recycle rocks. This is a lot to do with their durability; the interlocking crystalline texture of many igneous rocks make them impermeable and hard. The Whin Sill is one of the hardest with the stone being valued for use as setts and pavers as well as crushed stone for use as aggregate in tarmacked road surfaces. This durability means that when ice grinds its way across the land-surface (see previous blog “Scratching the Surface”), the Whin-stone resists the grinding better than the surrounding sedimentary rocks so that when the ice recedes the Whin Sill stands proud of its surrounding landscape.

Igneous rocks are valuable to geologists because they too can be dated and sourced, though this is where the analogy between ceramics and igneous rocks breaks down (if it hasn’t already!). Dating of igneous rocks is not about style but about geochemistry. The dependably regular decay of certain radioactive elements means that precise measurement of the ratios of parent and daughter elements can fix the time at which igneous rocks solidified. This means we can date the Whin Sill with a fair degree of accuracy to between 292-298 Ma. This not only gives us the age of the Whin Sill, but because it cuts across the Carboniferous sedimentary layers, we know that it was intruded after the sediments were laid down and turned into rock.

Twelve hours later and the kiln has been opened just as the pyrometer dips below 100^oC. The residual heat of the pots pushes up out of the kiln and with clumsy leather gloves the kiln shelves are lifted off to reveal the glossy
glow of the glazed pots. As the pots are lined up on shelves to cool, they start a gentle chatter of quiet ringing pings.  This happens as the glaze forms tiny cracks to allow the already hardened glaze to continue to contract. This process has its equivalent in igneous rocks; this is particularly noticeable in basaltic formations like the Whin Sill. Here the cracks don’t stop at the cooling edge of the hardened magma but propagate right the way to the middle of the intrusion forming distinctive joints. It is these vertical columnar joints which are eroding away on the side of Cullernose Point, the location of this month’s mystery rock, producing a series of ledges which are perfect for kittiwakes to nest on. 

@Northumbrianman

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