Glass melting crucibles

Determination of Hot Melt — Visually. Weigh accurately a 1 g sample of HBX into a tared sintered glass filtering crucible, medium porosity, 30-ml capacity. Using approx 5ml of ethylene chloride, divided into 5 equal portions, 1 min contact time each, extract sample on a Fisher filtrator with water vacuum. Collect filtrate in a 50-ml volumetric flask and dilute to the mark with ethylene chloride. Compare the color of the filtrate visually with previously prepd standards to det the weight of hot melt present. The prepn of the hot melt standard solns are described in 4.4.3.5.14... 

If the entire temperature dependence of viscosity is to be measured, it is necessary to use several methods based on different principles. In the viscosity range 10 —10 dPa s, use is mostly made of rotary viscometers. A platinum cylinder rotates around its axis in the glass melt in a crucible, and the force required for revolving the cylinder at a certain speed is measured. In another arrangement, the external crucible is rotated while the internal one is suspended on a torsion wire. Within the same viscosity region, it is possible to measure with a counterbalanced sphere viscometer a plat inum sphere suspended on a thin wire from the balance arm is immersed in the glass melt in a crucible. The other balance arm is loaded and the speed at which the sphere is withdrawn from the melt is measured. 

The study of fluoride melts basically requires the development of new experimental equipment, since the use of Al203-based ceramics leads to strong pollution of the studied fluorides by aluminium derivatives and glass-carbon crucibles are not suitable for oxide-solubility studies since carbon reduces the oxides formed from transition metals. 

ITie melting furnace consists of a ceramic melting crucible heated to a temperature of around 1150°C by five induction heaters. The molten glass is allowed to build up in the furnace for a period of 8 h, and then a glass plug in the bottom of the furnace is melted through the use of two additional induction heaters and the glass is poured into the stainless... 

In the original AVM design, an Inconel-crucible was used for glass melting, which requires more frequent replacement than a ceramic one. 

Sample Preparation. Two methods were used to produce sodium silicate glass samples for this study. The primary method used conventional glass melting techniques to produce compositions ranging from 12 to 21 wt. % Na20. Batch ingredients, African sand, sodiiam carbonate, and sodium nitrate, were melted at 1600 C for six hours in platinum crucibles, poured into patties and fine ground into 1 1/2" diameter discs with thickness of one to four millimeters. These anhydrous discs were fully hydrated in a one cubic foot autoclave under saturated steam conditions and stored in controlled relative hiimidity desiccators at room temperature. 

After the batch has been thoroughly mixed, it is melted at high temperature to form a homogeneous liquid melt. For small-scale melting of glass the batch will be placed in a crucible. For glass-melting experiments in the laboratory it is common to use Pt, or a Pt alloy such as Pt-5% Au or Pt-20% Rh crucibles. 

These crucibles are very expensive, but they can withstand high temperatures, they do not contaminate the glass melt, and they can be easily cleaned and reused. Other crucible materials, usually used for larger batch sizes, include silica, alumina, and mullite. 

The experimental set up is also equipped with a high resolution CCD camera. Pictures of the batch/glass melt in the vitreous silica crucible are taken at regular intervals. These images can be used for, for example, detailed observation of the reactions taking place, onset of melting phase formation, measurement of foam amount, and determination of the fining onset... 

The double crucible apparatus, which yields concentric bicomponent fluoride fibers, is a generic melt spinning process except that the dual melt is separately maintained under carefully controlled conditions in a supercooled state well below the llquidus temperature of the fluoride glasses [6-7]. The inner and outer crucibles form the concentric tip system through which the glass melts flow. 

It is well known that corrosion of refractories used in the superstructure of glass melting furnaces can occur due to reaction with components of raw batch (also known as batch carryover) such as silica sand and soda ash and also from vapor phase species, such as NaOH. While there is a standard test available from ASTM for corrosion of refiectories from vapors (C987), there is no standard test available for testing corrosion from batch carryover. The ASTM standard, C987, requires the use of either alumina or platinum crucible for melting batch components that produce vapors, such as sodium carbonate. For the purpose of this study, the authors chose to prepare crucibles directly from the fusion-cast AZS and vibro-cast AZS products. 

Each of these stages is perfonned more or less in sequence, either at one location, as in a crucible, or at a progression of locations, as in a continuous glass melting tank consisting of several, or many, zones. The various types of melters will be described in Sec. 6.3.3. Here we will describe glass-melting steps in more detail. 

---