THE ROLE OF WATER PRESENT IN THE BATCH ON THE FINING OF LEAD CRYSTAL WITH PbO CONTENT ABOVE 24 WT% M. RADA, J. VRŠOVSKÝ Institute of Chemical Technology Department of Glass and Ceramics Technická 1905/5, Prague 6, Czech Republic Tel: , Fax:

INTRODUCTION The redox state can be characterized by using the value of the partial pressure of oxygen physically dissolved in the glass melt. The partial pressure of oxygen represents an important thermodynamic parameter that is closely related with both the chemical composition (the glass alkalinity and the presence of polyvalent elements) and the glass temperature. Fincham and Richardson recognized that there must be three kinds of oxygen present in the melt: which becomes without the notation of the silicon atoms:

The so-called "alkalinity" of the glass is determined by the quantity of alkali oxides or the concentration of free oxygen (O 2- ). There is also equilibrium between the chemically and physically bonded oxygen. The physically bonded oxygen can be imagined as the oxygen in gaseous form that is bonded by weak van der Waals forces. The redox reaction of polyvalent elements in the glass can be expressed as follows: where Me... polyvalent element, O free, chemically bonded oxygen, O 2... physically dissolved oxygen. A simple reaction between water and the melt can be expressed as follows:

A structural equation characterizing the reaction of water with silica glass can be written in the following more visible way:

Project scopes The experiments were carried out with the aim to study the following problems by using a model lead crystal glass composed of % by weight SiO 2, 14 % ∑ M 2 O, 27.1 % ∑MO and 0.25 % As 2 O 3 : 1) The effect of the chemically bonded water on the fining process in batch samples of lead crystal containing PbO ≥ 24 % by weight. 2) The influence of the humidification of batch samples of lead crystal containing PbO ≥ 24 % by weight on the fining process. 3) The determination of the relationships between the water content in the batch and the nitrate content in the fining mix necessary for the optimum fining.

Experimental conditions There were two systems containing chemically bonded water: the PH system containing hydrated potash (K 2 CO 3 x 1.5 H 2 O) and the KOH system containing KOH. Other systems contained only the physically bonded water: the PC system with calcinated potash (water occurring only as a natural humidity of the raw materials), the systems with 2.5 and 5 % H 2 O (batches containing calcinated potash humidified to 2.5 % or, resp., 5 % H 2 O), designated as 2.5 % H 2 O or, resp., as 5 % H 2 O. In the above systems, the fining mix contained a constant amount of As 2 O 3 and the optimum amount of potassium nitrate was always looked for, i.e. the least possible amount of nitrates necessary for the achievement of the best possible investigated parameters.

As regards other investigated systems, no optimum KNO 3 - to-As 2 O 3 ratio was determined and only the values of some parameters were measured. A KNO 3 -to-As 2 O 3 ratio equal to 4 was used in the latter systems. The systems containing 1.25, 3.75, 7.5 and 10 % H 2 O were thus investigated (the batch containing calcinated potash was always humidified with the corresponding amount of H 2 O) - the designation is analogous to that mentioned above. The total K 2 O content (resulting from K 2 CO 3 and KNO 3 ) were kept the same during all the melting tests. The batches were prepared in such a way to yield 100 g of glass melt and they were melted in PtRh crucibles at a temperature of 1420°C for 112 minutes. If the batch had to be humidified, then an adequate amount of water was added to the well homogenized batch in the PtRh crucible from a buret immediately before the melting and the humidified batch was homogenized once again with a glass rod. The crucible taken out of the furnace after the heat treatment was quenched in cold water.

Parameters determined during the investigation The final degree of fining by a visual static method The surface tension by using the method of the maximum pressure in the bubble at the glass bath level The viscosity by means of the sink-point method

FIG.1: Dependence of the seed count in 100 g of glass on the ratio of KNO 3 : As 2 O 3

FIG. 2: Dependence of the surface tension at 1100°C on the ratio of KNO 3 : As 2 O 3

FIG. 3: Dependence of the glass melting temperature(°C)corresponding to the viscosity log  = 2 (dPas) on the ratio of KNO 3 : As 2 O 3

FIG. 4: Dependence of the seed count in 100 g of glass on the H 2 O content in the batch [ g ]

FIG. 5: Dependence of the surface tension at 1100°C on the H 2 O content in the batch [ g ]

FIG. 6: Dependence of the glass melting temperature(°C)corresponding to the viscosity log  = 2 (dPas) on the H 2 O content in the batch [ g ]

CONCLUSION 1)Water bonded chemically in batches containing KOH and hydrated potash worsens both the seed count per 100 g of glass and the values of the surface tension in comparison with the batch containing calcinated potash. 2)As compared with the batch containing calcinated potash, the humidification of the batch to 5 % H 2 O represents the best suitable option. The worst option is the batch humidification to 2.5 % H 2 O. 3)The optimum values of the KNO 3 -to-As 2 O 3 ratio for various batches are as follows: the batch containing calcinated potash - 4; the KOH-containing batch - 3.5, the batch containing hydrated potash - 5, the batch humidified to 2.5 % H 2 O - 7, the batch humidified to 5.0 % H 2 O )Regarded from the environmental point of view, the most important savings of KNO 3 can be achieved with the KOH- containing batch (12.5 %) and that humidified to 5 % H 2 O (25%).

5)Water present in the batch worsens the value of the seed count in 100 g of glass as well as that of the surface tension in the following sequence: the batch containing calcinated potash < the KOH-containing batch < the batch containing hydrated potash < the batch humidified to 5 % H 2 O < the batch humidified to 2.5 % H 2 O. 6)There is a direct dependence between the surface tension and the degree of glass fining. 7)The value of the surface tension is of pivotal importance for the fining process. 8)The effect of the surface tension on the resulting degree of glass fining predominates over that of the viscosity behavior. 9)The character and the water content in individual investigated potassium-containing raw materials is of a much greater importance for the determined parameters than the content of water physically bonded in the humidified batch.

10)The seed count in 100 g of glass and the surface tension at 1100 ° C increase with the increasing water content in the batch. Acknowledgement: This study was part of research programme MSM Preparation and research of functional materials and material technologies using micro- and nanoscopic methods