There are many methods of producing glass. However, the common method is melting at high temperature (about 1500oC). Generally, melting process includes 5 stages:

- Creating silicat

- Creating glass

- Homogenization

- Refrigeration

To reduce the melting glass temperature as well as improve the chemical properties and mechanical properties, Dolomite in form of dolostone (CaCO3.MgCO3) is put into the melting process. The roles of dolomite as below:

The roles of CaCO3

CaO is one of the basic components of glass. It makes the melting and reforming process easier, to make the glass withstand the chemical effects. However, high CaO could make the glass easily crystallized, brittle, and products require high annealing and steaming temperature. At low temperature, CaO could reduce the viscosity of glass. But at high temperature, the effects of CaO are complex. When CaO is about 10%, it could reduce viscosity, but it will has opposite effect if increase the CaO. Increasing the CaO will increase the gloss of glass. The reactions occur when have CaCO3 in production process:

Temperature [oC]

Reaction

< 600

Na2CO3 + CaCO3 = Na2Ca(CO3)

600 – 830

Na2Ca(CO3) + 2SiO2 = Na2SiO3 + CaSiO3 + 2CO2

630 – 930

Na2CO3 + SiO2 = Na2SiO3 + CO2

600 – 1000

2CaCO3 + SiO2 = Ca2SiO4 + 2CO2

Ca2SiO4 + SiO2 = 2CaSiO3

CaO content in glass oscillate in a defined range. In normal Sodium-Calcium glass, the CaO content is higher than 6%; packaging glass and bottle glass contain 7- 10% CaO; building glass contains 10 – 15% CaO. Impurities must be minimum, especially FeO. The Fe2O3 content in luxury glass is 0.03%, and in technical glass is 0.2%

The roles of MgCO3:

MgO put in up to 5% could effects materials melting easier, reduce crystallized ability of glass, and glass could be “longer”, annealing and steaming easier, in lower temperature glass there is only CaO content. The reactions occur when MgCO3 participate in producing process:

Temperature [oC]

Reactions when have MgCO3

<300

Create Na2Mg(CO3)2

>300

MgCO3 = MgO + CO2

340 – 620

Na2Mg(CO3)2 + 2SiO2 = MgSiO3 + Na2SiO3 + 2CO2

450 – 700

MgCO3 + SiO2 = MgSiO3 + CO2

>600

Rapid decomposition MgCO3

980 – 1150

Rapid reaction MgO + SiO2 = MgSiO3

CaSiO3 + MgSiO3 = CaMg(SiO3)2

Required dolomite must to achieve below standards:

Section

Content

MgO

>19%

CaO

>30%

FeO + Fe2O3

< 0,15%

Insoluble residue

< 2%