After the glass is melted in the furnace it is further conditioned and transferred to the forming section by a set of narrow channelas called the forehearth. Each forehearth has a superstructure which divides the channels into side channels and a central channel.

Channels work at constant temperature, thus thermal and chemical stresses, the temperature level and the glass corrosion at the operation temperature must be considered.

The temperature is the first factor to consider since it is limiting the application of some refractories. The glass corrosion is, actually, related to the temperature and this is why it is necessary to fix the temperature limit of the refractories (classification temperature) and choose the right ones for not reducing the normal campaign life.

Under 1270℃, high alumina (98-99 % Al2O3) sintered products in contact with glass are used, while at 1320 - 1330℃, zircon mullite bricks can be used. Zircon mullite brick is more resistant to glass corrosion than high alumina brick due to its particular crystal structure. If the operation temperature exceeds the above mentioned limits, fused cast AZS block must be used.

When glass is in contact with any refractory material at high temperature, the chemical reaction is a dissolution process. To increase the glass corrosion resistance, channels must obviously have the right porosity and density, but the crystal structure, especially the glassy phase content, is the most important factor.

The glassy phase content must be as low as possible because it is a weak point in a refractory material and to reduce it, clay must be definitely avoided. Alkali, infiltrating into the refractory during operation, react in particular with the glassy phase reducing its viscosity. Glassy phase exudes and carries away also the crystalline parts (zircon, zircon oxide, alumina) corroding and weakening the channel blocks. In short sintered refractories in contact with glass must have a crystalline structure and a very low glassy phase content for a strong glass corrosion resistance.

The operating factors of the forehearth superstructures are thermal, chemical and thermal stresses.The suitable refractories for high performances forehearth superstructures must have a low glassy phase content to resist both to vapor attacks and to creep.

Superstructure blocks are subjected to strong compression at high temperature and thus they must have high creep values. Creep is influenced by the content of low melting agents, which form the glassy phase, and this is why clay, carrying fluxing agents as sodium and potassium, must be avoided. Sillimanite brick is recommended here.

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