Porcelain covers with translucent pale yellow glaze?
This piece results from a very ancestral method: glazing using ashes, that what developed and exploited in Japan under the name of Nuka glaze. They used ashes as part of the composition of their glaze to create intricate changes in color and very surprising effect! Just google image it.
Indeed, ashes contain a lot of minerals, oxides like SiO2, Al2O3, Fe2O3, MnO, MgO, CaO… which are typically the oxides used in the composition of glazes. Depending on the material that produced these ashes, the composition is very variable. If only wood, they usually lack of color. There is no metal in wood. The content of iron, copper, cobalt… is really minimal. The picture above is a typical example.
The firing temperature also has a great influence. It needs many attempts to realise the right glazing. Too little temperature leads hardly to anything nice while too high temperature makes the glaze flow too much. You can nevertheless appreciate how smooth it looks! No cracks there. CaO is usually the oxide responsible for making the glaze flow.
Adding ashes on top of actual glazes creates lowers the melting temperature and can incorporate or not colouring effect. Best is to mix it well with an existing glaze composition.
The ashes were mixed with a dark green glaze and lead to this blue light color, rising probably from the large content of SiO2 and Al2O3 from the ashes.
Now ask your relatives to bring you ashes after the winter season, you will enjoy both their surprised faces along with surprising colouring effects!