The production of glass in large sheets or plates has been a historically significant part of the development of techniques for the manufacture of glass. The concepts involved are simple, yet the processes are fraught with pitfalls. Because of the requirement for flatness and clarity of the glass used for this purpose, great care must be taken in its manufacture. In addition, some of the processes may lead to uneven cooling of the glass, resulting in embrittlement and fracture. Several methods are available for producing plates of glass.

Blown Glass Plates (Broad Sheet Glass)

Broad sheet glass is the oldest technique for making glass plates. Although it is not a difficult process, the end result is of poorest quality when compared to the others. The manufacture can be accomplished with no more equipment than is necessary for normal glass blowing. A gather of molten glass is obtained on the end of a blowpipe and then blown to form a long cylindrical balloon. The glass is then removed from the blowpipe, the ends cut off and the resulting tube is cut along its length with a shears. The glass is placed on a flat tray and placed in an annealing oven to prevent uneven cooling which can result in embrittlement and fracture. The length of time in the annealing oven must be gauged carefully and is based on the thickness and composition (that is formula) of the glass. The resulting plate is typically quite small, and full of imperfections.

An improvement to this process is to concurrently blow the glass and swing it back and forth at the end of the blow pipe over an trough, allowing a longer cylinder to form. The resulting product is somewhat larger but of no better quality.

Blown Glass Plates (Crown or Spun Glass)

As with broad sheet glass, crown or spun glass is made using a blowpipe. A gather of glass is obtained on the end of a blowpipe and then blown in the normal manner. The glass is then attached to a pontil rod at the end opposite to the blowpipe, which is then removed. The pontil rod is then spun along its axis, causing the glass to flaten into a disk. Then the glass is set on a flat surface and removed from the pontil rod. This process leaves a central blemish in the glass, and fine ripples in the glass not unlike those caused by tossing a rock into a pond. If not properly done, it also creates an unequally round plate of uneven thickness. The size of the resulting plate is significantly larger than the product of the broad sheet process. While the glass is still warm, plates are typically cut from the areas away from the center, where the pontil has left its mark. As with broad sheet, the resulting plates are placed in an annealing oven to prevent uneven cooling.

Cast Glass Plates (Polished Plate)

The process of producing cast glass plates involves pouring molten glass into a low flat mold. The glass is allowed to cool until it is no longer molten, and is then removed from the mold and transferred to a flat tray. The tray is then placed in an annealing oven as with the blown glass plates. Once cool, the resulting plate is polished on both surfaces to remove blemishes.

The wet polish process is performed using successively finer polishing grit. Water is poured on the surface of the glass is, and a powdered grit is sprinkled upon it, then a leather buffing pad is used to scour the surface. For the best success in the buffing process, the leather pad should be stuffed with a soft and compliant material, allowing the grit to adhere to the pad, and not merely be moved around the surface of the glass. The buffing should proceed in a small circular motion to improve the overall performance of the grit. Once the more coarse grit has sufficiently removed the larger imperfections in the glass, the buffer pad and the glass should be rinsed, and finer grit applied with the buffer. This process is repeated with successively finer grit polishing powder until the glass is smooth and without surface blemish. The glass is then turned over and the buffing process repeated.

Despite of the polishing process, the resulting product is typically of poor quality as a result of internal imperfections in the glass.

Cast Glass Plates (Rolled and Polished Plate)

The second cast glass technique involves pouring molten glass onto a flat surface, and rolling it smooth. It is preferable to use a polished metal surface called a casting table, as wood will burn, and cause bubbling and staining of the glass. The process of pouring the molten glass onto the flat surface is in itself dangerous because of the potential for splashing the molten glass. In addition to the danger, the rapid change in temperature of the glass on striking the surface of the mold can result in uneven cooling and possible fracturing of the material as it cools. In order to reduce the probability of this occurrence, it is desirable to preheat the mold before the glass is poured.

Once the molten glass has been poured onto the flat surface, a large metal roller is brought to the surface and carefully drawn from one edge to another, in a manner similar to that of rolling a pie. This step must be performed cautiously, as the glass should be kept even across its lateral dimensions. To keep the height constant, runners can be placed to either side of the glass.

Once the plate has been smoothed and flattened sufficiently, it must be transferred on a flat tray to an annealing oven large enough to take it fully inside for proper, gradual cooling. Alternatively, the glass could be cut into smaller plates prior to being placed into the annealing oven.


OOC References:

Making Glass: From Craft to Mass Production
http://primarysources.msu.edu./curricula/artifacts/glass/

London Crown Glass Company - Glass for Period Building
http://www.londoncrownglass.co.uk/index.html

PFG Glass Company - The History of Glass
http://www.pfg.co.za/history.htm

The British Glass Company - Making Glass
http://www.britglass.co.uk/

And for a bit of humor.

Urban Legends - Glassmaking
http://www.urbanlegends.com/science/glass.flow/glassmaking.html

A word or two on the process of annealing

A process for treating glass to make it less brittle and more resistant to fracture. Glass cools quickly on the surface leaving the core much hotter, which causes severe internal stresses. Annealing removes these internal stresses and also minimises internal defects in the material.

Glass is annealed by heating it to just below the softening point and then slowly lowering it to to room temperature. Annealing time varies widely according to the thickness of the individual piece glass, and for thick plates of glass may require requires several hours, or even days.