I said it many times before, this is my favourite glaze since the very beginning. The Chinese called it Sang de Boeuf, blood of the ox. It is a bit tricky to achieve. First of all you need a kiln with and open flame, fired with gas, oil, wood. The reason for that is, that you need to take oxygen out of the atmosphere during the firing. Then the oxygen-hungry flame takes the oxygen from the copper-oxide within the glaze. This leaves a layer of metallic copper, which gives this lovely colour. I once read an article where they had microscopic images of a shard of a copper red pot. You could see 3 layers: first a layer, in which clay and glaze melted together. This is normal for stoneware and it is the reason why it is so sturdy, durable and frost proof! It also means that the clay-body gets soft and is able to melt together with the glaze. Second layer was a fine layer of metallic copper. And the third layer a clear glass, resulting in the shiny surface of the pieces. When I fire the gas-kiln I take 3-4 hours to go up to 960 degrees C, then I close the damper over the flue exit to leave only about a centimetre space. This reduces the influx of oxygen and the kiln goes into reduction. I can see it because the hot gas comes out of the spy-holes and ignited when it comes into contact with the air/oxygen. The trick is then to keep the temperature rising steadily up to about 1270 degrees C. This takes about 6 hours, by which time I will have been sitting next to the kiln for about 10 hours. It is exiting every time! The red colour develops when the kiln cools down to about 800 degrees C (that was also in the article). Don’t ask me how they found that out!!!!!!
So every time I see a copper-red piece I also think about the process it has been through to be soooooo beautiful. Some of my mini poppy seed heads are copper red, you can find them here https://www.ceramicsbuyanja.co.uk/product/poppy-seed-heads/ and choose the copper-red option. This is a good book on the subject with a bit of science in it by Robert Tichane https://www.amazon.co.uk/Copper-Red-Glazes-Robert-Tichane/dp/0873416635/ref=cm_cr_arp_d_product_top?ie=UTF8