Flowers on grass? Monet painting? Pastel colors, round shapes. A few shades in the pale green, very subtle. On the parterre, blue flowers, maybe pansies? randomly scattered. They have a distinctive lighter contour and the petals emerge from the base with darker shades of violet-blue. The faint lines from the center of the pansies toward the outside convey the fragility of the petals, as if they were delicately placed above the greenery.
This upper part of a stoneware pot has been glazed with a crystalline glaze. It is a very liquid suspension containing tiny crystals of zinc oxide ZnO, of few nanometers. As the furnace is heated up, the glaze gets very liquid and atoms from other elements in the suspension, silica Si, copper, Cu… , can diffuse and attach to the small zinc oxide crystals. This is a nucleation process.
Depending on the chemical elements available, various crystal structures can form. We call them classes of crystals depending on the symmetry in the arrangement of the atoms: monoclinic, cubic, hexagonal, etc. For example, for a cubic crystal, the atoms are placed at the edges of a cube. When the crystal grows, it remains a cube, unless there are some impurities like a bigger atom, that will bring disorder and reorient the growth in another direction.
The growth takes place when the glaze starts cooling down. The slower the cooling, the bigger the crystals. In this particular case, the cooling rate was not controlled, but after some trials I found that 1235°C for 10 min was giving the best results. Next I will try 1230°C to see if the crystals are bigger. It is usually difficult to control the cooling rate of a furnace as it would require a cooling system, mostly by the circulation of air or water. Cooling too fast would endanger the thermal resistance of the parts in the furnace, as well as the ceramic, and lead to cracks and deformed elements.
The crystals formed by zinc, silica and the other components are typically called willemite, of formula Zn2SiO4, which has a hexagonal structure. But perfection is not natural therefore they grow as a spherical shape. The additives such as copper or manganese will get incorporated within the crystal structures and give colors to the crystals.
The beauty of the process raises from the uncontrollable flow of the glaze high temperature. It leads to areas (usually at the bottom) with more nanocrystals and therefore more macroscopic crystals at the end. The shape of the brush and the mixing of the glaze have their importance too. It took me quite some time to find the right temperature profile: a different result for a different oven, a different results for another place in the oven!
Let’s continue to be surprised!