Glass and Glass Manufacture

by Percival Marson

The extremity of this cylinder is now re-heated and opened with the aid of a spring tool with charred wooden prongs, until the opening is enlarged and drawn out to the same diameter as it is throughout the cylinder. It is now in the form of an open-ended cylinder (e).

The cap of the cylinder at the blow-iron end is now cracked off. A thread of hot glass is wrapped round the shoulder near the cap, and the line chilled by using a curved, hook-shaped rod of iron. Whilst the cap is being cracked off, the cylinder is allowed to rest supported by a wooden cradle.

The cylinder is now open at both ends (f) and is taken to the flattening kiln or furnace. This kiln has a level, smooth floor, heated from below, upon which the cylinders are flattened out. Placing the cylinder on the floor in front of him, the workman places along the inside length of the cylinder a long red-hot iron rod touching the glass, and then chills the line with a touch from a cold iron rod. This causes a split to take place along the whole length of the cylinder. As these cylinders are split open, they are removed to a hotter zone within the flattening kiln, where the heat causes the cylinder to uncurl and gradually flatten out.

As the sheet becomes flat the workman levels it out with a flat block of charred wood called a polisher. This is attached to a long handle, and is rubbed over the face of the sheet of glass. The weight of the wooden block is just sufficient to smooth out any creases and assists in levelling out any irregularities of the surface. It is essential that the floor upon which the glass is resting should be perfectly smooth and level, and uniformly heated. As each sheet is levelled, it is removed to the annealing oven and afterwards stacked up until cool, after which the rectangular sheets are cut up to the various sizes required for window panes.

It is evident that the crown glass method gives more waste in cutting up, and does not provide such large sheets as the cylinder method. On the other hand, cylinder glass always shows a certain amount of waviness on the surface, and is not so brilliant as crown glass. The better surface of crown glass no doubt is due to the fire-polishing it receives when being expanded out into the disc. It appears to be somewhat difficult to get a perfectly smooth level face to cylinder glass by using the wooden polisher.

Plate Glass is used as mirror glass and in glazing shop windows and showcases. It may vary between 1/4 and 3/4 in. in thickness, and is more expensive to produce than crown or cylinder glass.

In the manufacture of best plate glass, the materials are melted in open crucible-shaped pots of varying sizes; sometimes, in making large, heavy plate, their capacity reaches 25 cwts. of metal. When the metal is plain and clear from seeds it is either ladled out into smaller crucible pots for casting, or, as in the case of casting large sheets, the whole crucible of metal is lifted bodily out of the furnace by means of a crane, and, after being skimmed, is conveyed by an overhead travelling derrick to the casting table.

This table is a level iron bench the size of the plate to be cast, the face of which consists of thick sheets of iron plate riveted together to form a level top; along the whole length of each side of this table is a raised flange of a height sufficient to give the thickness of the plate of glass to be cast: resting on these two outer edges a long, heavy metal roller runs, covering the full width of the table. The crucible of hot metal is brought to a convenient position and the contents poured out on the table in front of the metal rollers. These rollers then travel along and squeeze or roll out the hot metal over the surface of the table to the thickness regulated by the side pieces, which also prevent the metal from flowing over the sides. The empty crucible is then conveyed back to the furnace for refilling.

The cast plate of glass is then trimmed from any excess of glass at the ends, and when set and stiff is lifted at one end slightly and pushed forward into a conveniently situated annealing oven, where it is re-heated and subjected to a gradually diminishing temperature to anneal it. The plate of glass, as delivered from the annealing oven, shows surfaces somewhat rough, wavy, and uneven, from the marks left by the table and the roller, and it has to be ground and polished level and smooth on both sides. This is done by fixing one face of the glass plate in a plaster of Paris bedding and setting it within a mechanical grinding machine.

This machine carries several revolving arms, to which are attached other smaller plates of glass. These are used as the rubbers, a slurry or paste of sharp sand and water, or abrasive powder, being interposed between the two. The revolving circular motion of the arms causes a grinding action between the two plates, which wears down any irregularities and gives a more even face. After this, finer grades of abrasive materials are employed, and, finally, polishing powder, until the face of the glass plate is polished smooth and level. The large plate of glass is then reversed and the process of grinding resumed on the other side.