Glass Melting in the Laboratory
The starting raw materials for small-scale glass melting in the laboratory include inorganic oxides (e.g., SiO2, Al2O3, Fe2O3), salts (e.g., Na2CO3, CaCO3, Na2SO4, Ca posphates, NaCl), and their hydrated compounds (e.g., H3BO3, borax, Al(OH)3). The starting materials are mixed according to the desired stoichiometry, forming the glass batch, which transforms to a glass melt at elevated temperatures. A simple glass batch calculator can be downloaded here (300 kB), based on the technique described at Wikipedia.
When selecting the batch materials, attention must be paid to their stability during storage. Air-sensitive chemicals should be avoided because they might already have reacted to other compounds that would cause the glass composition to be different from the one intended. In particular, alkaline earth oxides MgO, CaO, SrO, BaO and certain dehydrated salts are not recommended as batch materials due to their reactivity with water and carbon dioxide from the atmosphere. Other materials, e.g., iron compounds with Fe in the oxidation state 2+ might have changed the oxidation state.
In addition to the stability of the batch materials during storage their evaporation behavior during heating up to melting temperatures must be considered. For example, water-containing boron compounds such as boric acid and borax (Na2B4O7*10H2O) tend to evaporate more easily than dehydrated boron compounds like boron oxide and kernite (Na4B2O7*4H2O). Similarly, SeO2 evaporates easily during batch melting, while sodium selenite evaporates less.
The transformation from the batch to the glass is a chemical reaction. Some compounds react more readily than others, e.g., alumina is often easier introduced in glass using Al(OH)3 compared to Al2O3. Small particle sizes in the batch facilitate melting.
Further instructions concerning the batch material preparation can be found on a website of the University of Washhington.
Common silicate and borosilicate glass melting in the laboratory is best accomplished in platinum or similar noble metal crucibles. Other crucible materials often get partially dissolved in the glass, therefore, they are only advisable if a good glass homogeneity is not required, of if the noble metal reacts with certain special glass compositions.
During heating up of the batch often a high volume a gases develop. Therefore, the crucible should be charged initially only with a small fraction of the batch to prevent a boiling over the crucible edges. The remaining batch can be added stepwise.
A good glass homogeneity is acheived by a good mixing of the batch in a mill, by stirring of the melt, or by crushing and re-melting of the first melt.