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Glass Melting in the Laboratory

Starting Materials

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 materialsare 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 behaviorduring 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.

Glass Melting

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.

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Melting in the Laboratory http://glassproperties.com

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