their batch into an annealer and raised the temperature to 500-700° F. The hotter charge will lower your heat differential and cut your melt time.
The amount and frequency of charging is another key element. Small charges minimize the degree of temperature drop and the duration of time the contact surface remains at the reduced level. The amount of the charge is dependent upon your operation, the length of time you allot for the filling sequence, and the size and configuration of the crucible. A shallow, wide crucible affords the best receptacle for charging; it affords the least amount of wall space for potential material contact. It is recommended that the amount of each charge be limited to about one fourth of the volume of the crucible.
The interval between fills should be of a duration that allows the interior wall of the crucible to return as near to temperature equilibrium as possible. The minimum point for this occurs when the pile of the charge softens and flattens out. The sequence is repeated until the crucible is filled. The increased time between charges ensures a greater equilibrium between the inner and outer walls.
The next area of concern is crucible corrosion resulting from the batch charge as well as the relative corrosiveness of the batch glass itself. Keep in mind that the lower the viscosity of the glass the faster it will attack the liner. The higher the alkali and lithia the more corrosive the glass. Lithia is very popular as a batch addition because, in small amounts, it decreases glass viscosity appreciably.
Temperature is an important contributor to influencing the effects of corrosion upon refractory surfaces of a crucible. Temperature is an accelerator; its excessive use in combination with the alkali and other aggressive components in glass will destroy or disrupt the contact surface of the crucible to a point whereby unsatisfactory properties such as stones, seeds, and cords are imparted into the glass. Melt at the lowest temperatures possible because excessive temperatures will ultimately compound your problems. The correct temperature has to be determined by each operator; it is dependent upon factors related specifically to your operation, such as batch composition, desired glass properties, furnace design, etc.
The use of batch, cullet, or combination is a factor
for consideration. The easiest to melt and handle is cullet; however, glass quality, flexibility, and control are not as good as that obtained from batch. If price is a factor, cullet may be cheaper to use. When you factor in the shipping costs of cullet, however, batch may prove to be a viable alternative. Making your own batch can be very problematic and dangerous if the right precautions are not taken. Chemical storage can be a problem. Conversely, if you understand the care and handling of these chemicals, the resultant glass quality and flexibility will be far superior to cullet. Cullet has two great advantages over batch: it has an unlimited shelf life, and it won’t attack your crucibles quite as harshly as batch will. A crucible under optimum
conditions will have a life expectance of about 90 - 100 campaigns, possibly longer when using cullet.
It is important to thoroughly mix the batch prior to its addition into the crucible—this assures homogeneity in the melt, and it reduces the tendency of destructive corrosive elements of the batch to come in direct and sole contact with the refractory surface of the crucible (resulting in pitting and subsequent related glass defects). If you are able to pelletize your batch or purchase a pelletized batch, you will be able to minimize the corrosiveness of alkali because it will be encapsulated. Heat penetration of pelletized batch is very good and allows a more rapid temperature equilibrium. A similar effect can be enhanced in batch by adding pea-sized cullet.
The proper position and placement of the fill inside the crucible is important. It is not a good idea when charging batch to leave a mound in the center of the charge as the alkali will melt and run down to the periphery of the charge where it will then contact the crucible and “eat” into the surface. Flatten the charge to prevent this from happening. It would also aid in the protection of the crucible if a glass residue, referred to as “heel,” could be left in the bottom of the crucible. Heel serves as an insulator between the surfaces of the crucible and the elements of the charge. It protects the bottom, is pushed to the outer surfaces when new, and flows upward to offer a protective barrier between the effects of the batch and the refractory nature of the crucible. Proper application of these practices provides favorable results in several areas: it reduces
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