70 | Glass Manufacturing Methods, Molding Methods, and Processing Methods 8

Hello everyone 🎵

Tokyo continues to experience unusually intense heat, with sudden downpours in the evenings, and earthquakes in Ibaraki, Kanagawa, and Kyushu. There's no time to relax. First and foremost, health is paramount. Please make sure to drink plenty of water, be careful about heatstroke, and maintain your physical condition by ensuring proper meals and sleep.

I took a week off for the Obon holidays, so it's been a while, but I'd like to continue discussing the manufacturing process of glass.

Last time we covered raw material blending, so this time we'll talk about melting.

The powdered raw materials are blended and mixed (this is called "batch") and then melted at high temperatures. Before shaping, glass is heated and melted to about 1400°C, and it solidifies when it cools to about 600°C. During this process, capturing the precise moment when the glass is at its most beautiful and shaping it is key to creating excellent glass.

The equipment used to melt glass is called a "furnace" (kama). There are two main types of furnaces: "crucible furnaces" and "tank furnaces."

A "crucible furnace" is shaped like a cat sitting, as it's commonly known as a "neko-tsubo" (cat pot). About eight crucibles are arranged in a circle, with their openings facing outwards, where raw materials are placed and melted at high temperatures. This type of furnace operates continuously, 24 hours a day, 365 days a year. In handmade glass factories, there are craftsmen called "kamadaki" (furnace tenders) who manage the melting process. It's a very demanding job with continuous day and night shifts, but it's no exaggeration to say that the quality of the finished glass largely depends on the skill of these kamadaki craftsmen and the quality of the glass they produce.

As the manufacturing day draws to a close, the furnace tender prepares to put raw materials into the crucibles. Instead of just melting raw materials, adding cullet (which we'll discuss in detail in the shaping process), the excess parts left after making glass, lowers the melting temperature and increases thermal efficiency.

The crucibles containing raw materials and cullet gradually heat up from evening, becoming a thick, syrupy liquid at 1500°C. By morning, if the glass is still in a syrupy, molten state, it's too soft for shaping. So, the temperature of the glass is lowered to a shapable temperature, prepared to the optimal temperature for the craftsmen when they arrive for work and begin shaping.

Since raw materials are in powder form, melting them at high temperatures can trap air between the particles, forming bubbles. If there are bubbles in the glass, its value as a product decreases. Therefore, the furnace tender devises ways to minimize bubbles by adjusting and controlling the furnace temperature.

Also, if the amount of glass decreases during daytime shaping, the condition of the glass changes. Changes in the glass's condition affect the shaping process and the finished product, so the furnace tender must remain vigilant throughout the day.