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The Care and Feeding of Crucibles

    Reprinted with permission from the book, Glass Notes by Henry Halem. Permission is extended to all who wish to download and reprint this information.
One of the most common complaints from people using pot furnaces is that the crucible that they paid big bucks for cracked after being used about one week. There are many reasons why crucibles crack before their time. The following information concerning the care of crucibles was supplied and written by John Bartel of The Laclede Christy Crucible Company.
        The change of the glass melting crucible composition from the clay flux type to the more resistant, high alumina, zirconia materials, has introduced a new factor into the successful thermal management of the crucible. This factor is referred to as "down shock" - a phenomenon wherein the differential temperatures between the inside wall of the crucible and its outside surface are of sufficient differences to develop a destructive stress force which will result in cracked refractory crucible walls. Normally, these cracks are not destructive to the point they render the crucible useless, but they do establish a condition whereby corrosive forces associated with glass materials, have a more favorable environment for accelerated deterioration in the crack zones. Ultimately, this results in a destructive hole through the structure of the crucible. The cracking described accounts for the majority of premature failures There are other factors which can cause this type of cracking, but the largest contributor is the manner in which the filled material, batch-cullet is added to an active crucible. The addition of batch, cullet, or combinations thereof introduces the mechanism for lowering the interior surface temperatures of the crucible. The amount, frequency, and temperature of the charge are the key contributors to the development of forces severe enough to crack the crucible's surface. The addition of clod charge, in contact with the walls of the crucible, can drop the temperature, in the area of contact, to 1000 °F (537 °C) in a matter of a few minutes. This results in the interior wall, or that portion of contact, wanting to shrink while the other, hotter surfaces want to expand. It is this heat differential between the inner and outer wall that creates enough stress to sometimes crack the crucible. There are ways to moderate these forces.
1. The temperatures of the charge material should be as warm as your operation will permit. In addition the material must be dry. You can help yourself if, at the very least, you bring the material in and allow it to sit in the warm surroundings of the furnace prior to its introduction into the crucible. Some people place 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.
2. 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 fiattens 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. The higher the alkali the more corrosive the glass. There are other glass chemicals that attack refractories as well. The most active of these is lithia. As many of you know, lithia is very popular as a batch addition because, in small amounts, it decreases glass viscosity appreciably. When making lithia additions, it is not wise to go above ¼" of 1%. Some key points when melting are as follows:
1. 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.
2. 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. I know a number of glass artists who are able to maintain their crucibles for well over a year. (Would you believe 9 months?)
3. 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. The problem with the Littleton pelletized batch is that it has a tendency to break down during shipping and gets somewhat un-pelletized. 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.
4. The proper position and placement of the fill inside the crucible is important. Where possible, it should be placed in such a manner so that it peaks in the center of the crucible and away from the walls. 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 thermal shock influence and minimizes the corrosive attack on the crucible.
5. The use of cullet presents its own set of problems when charging into a crucible. When charging cullet, it is wise to avoid throwing large, cold, heavy chunks into the crucible. You should not fill the crucible to the top with cullet because the temperature differential will crack the pot for the reasons explained earlier. If you follow the directions outlined, you will minimize your risks.
In reality, there are few differences in the effects upon a pre-fired refractory oxide crucible, whether you are using batch or cullet. The majority of crucible failures occur for all the stated reasons (as well as others that are impossible to cover) as related to the placement of cullet and batch materials into a crucible. The problems and possible solutions stated above seem to be the ones that affect the studio artist the most and should be followed as closely as possible.
        Crucibles are not indestructible. Proper furnace management, common sense, and patience will provide you with an economical medium and quality glass.

Summary

1. When lighting up, start with a small, soft flame and build the temperature gradually until you get above red heat. At that point, you can raise the temperature more quickly. Many crucibles crack during start up. Crucibles are like a piece of glass: if you heat them too fast they crack; if you cool them too quickly they can also crack.
2. Use dry, warm, and, if possible, preheated material.
3. Make multiple fills in small quantities, allowing adequate time between additions for temperature equilibrium.
4. Pile or peak fill material in the center of the crucible, away from contact with the wall.
5. Keep a residue, or heel, amount of glass as an insulator against initial fill addition.
6. Use a percentage of cullet with raw batch if possible. Your glass will melt faster.
7. Minimize the use of temperature. Excessive temperatures create excessive crucible wear.
8. Make sure your batch is thoroughly mixed and not agglomerated. Meaning, if you have a clumps of soda ash, they will deteriorate the crucible at the points of contact when they melt. If soda fluxes sand, it will definitely flux your crucible.
9. Keep tramp metals and foreign materials out of cullet or batch. Metals can drill holes through crucibles.
10. Remove glass from the crucible and reduce the temperature if the operation is placed in an idled or holding pattern for a prolonged period of time.
There are other reasons for pots breaking in addition to the ones stated. Of course you can crack a crucible if you hit the pot with your blow pipe or a mongo ladle. It is also possible that your pot was cracked before you put it in the furnace. Check your pot carefully, it may have cracked in shipping. If it has, refuse the shipment and fight it out with the shipper. Do not store pots inside each other to save space. The weight of the top pot will crack the one below. If your pot happens to crack after being used one week and you call the company, they'll give you a hard time. If your pot suddenly springs a leak because of tramp iron in the pot mix or voids created during manufacture, then call the company and fight it out. I have heard stories about poor manufacturing standards. A void is air trapped in the pot mixture during manufacture. You cannot see it until your glass drills into it and leaks out for no apparent reason. Tramp iron is a piece of iron that has gotten into the mix and, when heated, drills a hole through the pot. Pots also crack due to thermal gradients in the pot furnace itself. Don't put a cold pot in a hot furnace. Again, don't drop huge chunks of cullet into the pot. Use common sense; pots are very fragile when they are hot. The bottom line is: baby your pot, heat it slowly, and cool it slowly, and don't use a very active glass. High percentages of lithia in your batch will eat your pot like pac-man. Your pot may last a year if you're lucky. Very lucky. Some glass blowers swear by Laclede Christie and others by Ipsen. You pay your money and you take your chances. It is a fact, though, that freestanding crucible furnaces are a lot more economical than day tanks. The advantage to day tanks is that they can take a great deal of abuse. If you do a lot of sand casting and use big heavy ladles, you might consider building a day tank.
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