Shotoku Glass

松徳硝子株式会社

History

Established as a manufacturer of light bulbs in Tokyo in 1922, Shotoku Glass was originally renowned for their niche technique in mouth-blown glass. As the light bulb industry automated, Shotoku Glass shifted to manufacturing glassware, concentrating on the production of mouth-blown glasses in several thousand varieties.

Molten Glass

All Shotoku glasses are made using the method of mould-blowing: blowing air into molten glass, and using metal moulds to create products of the same shape. A piece begins when the glassblower extends their blowpipe into the glass furnace to gather a layer of molten glass. Once ready, the blowpipe is withdrawn and air is blown through the pipe to form the glass into a small ball.

Blowing into shape 

A skilled glassblower then extends the blowpipe, with the molten ball of glass, back into the furnace, rolling it round and round to gather the appropriate amount of glass depending on the eventual size of the product to be blown. The molten glass is withdrawn and placed inside a bowl-shaped mould, after which the glassblower lifts the blowpipe upwards and blows air through it. This process requires long experience and distinctive design skill.

Slow cooling down and inspection
To prevent the mouth-blown glasses from cracking or breaking due to rapid cooling, they are instantly placed into a special glass kiln called a lehr that operates at a starting temperature of around 500 degrees Celsius. The glasses move slowly through the lehr on a conveyor belt, gradually being cooled down to room temperature. Every piece is subject to precise quality control.

Burning and cracking off
A straight line is first etched into the glass with a diamond wheel cutter to the required height. The glass is then set on a rotating base, and is exposed to the burner flame along the straight line previously etched. Being specifically heated along the line allows the excess glass above, where the molten glass was once attached to the blowpipe, to be cracked off by hand from the body.

Sanding off and polishing
The remaining sharp edge of the glass is then ground down and sanded off, by machine and by hand. After first using a mechanical glass sander, a skilled artisan will grind the rim of the piece by hand to create a perfectly smooth edge. To do this, emery powder and water mixture is kneaded together and placed onto the centre of a rotating steel plate. With centrifugal force, the emery powder and water is uniformly and thinly spread on the plate, allowing greater consistency and a smoother polish to each piece. 

Reheating of the rim
After the glass edge is sanded down, it is washed ready for the final process. The top edge of the glass is reheated with a burner flame until it is semi-molten; this completes the glass with a fine finish against the lips. Especially due to the thinness of the glasses, a rim that is reheated too much will cause distortion, while insufficient heating will result in an uncomfortable edge of the glass. As such, this process essentially determines the quality of the finished product. 

The factory that never stops
The furnace operates continuously, 24 hours a day and 365 days a year, always at a temperature of between 1,300 to 1,500 degrees Celsius. At 17:00, when artisans finish their work, the craftsman who specializes in glass melting arrive to carry out the process of glass melting throughout the night, every night.

Finished product

A set of ultra-thin red wine glasses, an example of one of the many glassware products made by Shotoku Glass. Find these and other ultra-thin glasses by Shotoku Glass at Japan House London.