The phase stages of glaze formation during the firing process
ـThe glaze used on the bodies undergoes its phase transformations during the firing cycle according to the following steps:
– The formation of silicates
– Glass phase formation
– Refinement phase
Feldspars, in the composition of the glaze, introduce alkalis as network modifiers, aluminum as a stabilizing agent, and SiO2 as a glass former in an insoluble form. During the first phase, i.e. the formation of silicates, what happens is solid phase reactions. The first solid phase reaction is the modification of beta quartz to alpha quartz at 573 degrees Celsius, which takes place spontaneously.
Another important component used in the glaze is CaCO3, which breaks down according to the following reaction:
CaCO3 CaO + CO2
Following the above reactions, the formation phase of silicates begins with the formation of calcium silicate:
2CaO + SiO2 2CaO.SiO2
CaO + SiO2 CaO.SiO2
The formation phase of silicates ends around 900 degrees Celsius, while the glaze still contains the remaining quartz in its structure. Eutectic melts are the first molten phases that are formed during the increase in temperature and in the temperature range of 700 to 900 degrees Celsius. In the second phase, the reactions of glass formation take place. Most of the remaining free quartz enters the molten phase during this stage of the baking process.
At the thermal peak of glaze firing, the standard oxide components used in glazes, namely CaO, MgO, K2O, Na2O, ZnO, Al2O3 and SiO2, are all present in the molten phase. While the glazes are in the state of the glass phase or sometimes in the next stage of purification, they harden during the cooling stage.
In any case, the complete melting process of glass during the standard firing cycles of ceramic products is not always certain. Therefore, we see the presence of needle holes throughout the structure of some glazes, which indicates the presence of undissolved particles in the melt (SiO2) glaze. A transitional area is formed between the glaze and the body, which is called the middle layer or buffer layer, and it has a higher glass phase content compared to ceramic bodies.
Depending on the specific chemical composition of the buffer layer, sometimes we may encounter the formation of crystalline phases. In glazes rich in CaO, the crystallization of anorthite and the presence of cristobalite can be proven with the help of scanning electron microscope analysis. The effect of alkaline feldspars (preferably feldspars rich in alluvite) during the stages of glaze formation is mainly seen in the second stage, i.e. the formation of the glass phase. These feldspars start to melt at 1120 degrees Celsius and expand the glass structure of the glaze in connection with other oxides.
In its simplest form, this structure consists of alkaline earth-alkaline aluminosilicate glasses. Adding Al2O3 with the help of feldspars has the advantage that its dissolution and entry into the molten phase can be done almost without problems.
The hypotheses that are used to explain the structure of silicate glasses are also valid in describing the processes that occur during the formation of the glaze glass phase, and the results show the necessity of the presence of alkalis and alkaline earths, Al2O3 and SiO2 in the batch formulation version. Glaze shows.
Alkaline-alkaline earth aluminosilicate glasses that develop during the formation of the glass phase include irregular connections of [SiO4]- tetrahedrons, which basically includes 60% of connections. Al2O3 has the ability to replace SiO2 in the network and therefore, in the presence of alkaline oxides, it appears in the form of tetrahedral coordinates such as tetrahedron [AlO4]-.
Adding feldspar to the glaze version guarantees the provision of all three mentioned oxide components. Both alkaline and alkaline earth ions act as network modifiers. Although Al2O3 can act both as a forming agent and as a network modifier, its behavior depends on the acidity or alkalinity of the glass phase.
As the temperature increases, more alkaline [Na2O, K2O] and alkaline earth [CaO, MgO] enter the glass phase, which in turn increases the alkalinity of the glass phase and therefore makes possible the formation of [AlO4]- tetrahedrons. [SiO4]- and [AlO4]-tetrahedrons form a glass structure, which is considered a glaze glass network based on the mentioned model.
Alkaline and alkaline earth ions – as network modifiers – are connected to tetrahedrons through oxygen. This mechanism causes bridges to break and destroy the network structure. Increasing the content of alkaline and alkaline soil oxides increases the number of fractures. [AlO4]- tetrahedrons reduce these fractures. This effect of Al2O3 continues as long as sufficient alkaline and alkaline earth ions are available to balance the capacity of triple positive aluminum ions [Al3+] in [AlO4]- tetrahedrons. With this description, we conclude that a batch of glaze is regulated when it contains an appropriate amount of components that form and change the network in order to form a glass structure.
By: Omid Holakoue
