The onset temperature of fining and the quantity of fining gases is not only determined by the amount of fining agents in soda-lime-silica batch, but also by the level of organic contaminants in the cullet or normal batch and the water vapor pressure in the furnace atmosphere. These conditions will also determine the redox state of the glass and residual sulfate or sulfide concentrations in the glass. Organic contaminants will form char during heating of the batch. This char partly reacts with CO2 coming from the decomposition of the soda, limestone or dolomite forming carbon monoxide. Stable types of char or cokes or cullet-rich batches with only small amounts of carbonates will result in some carbon residues after CO2 evolution. This carbon partly reduces sulfate and ferric iron in the fresh melt which results in sulfide & ferous iron formation in these glass melts.
