About the Sheet Glass Manufacture: The Float Process Reading Passage

This passage explains how high-quality flat glass is manufactured using the float process, a method developed by Sir Alastair Pilkington. It covers the composition of raw materials, the role of a tin bath, the controlled cooling process called annealing, and quality-inspection techniques. The passage comes from Cambridge IELTS 7, Academic Test 1, Passage 3.

You should spend about 20 minutes on Questions 1–13, which are based on the passage below.

The passage contains two question types:

• True/False/Not Given (Questions 1–7)
• Diagram Labelling (Questions 8–13)

Paragraph A 

Glass is made from a batch of raw materials like silica sand, soda ash, dolomite, limestone, and a small proportion of other materials. On arrival at the glass-manufacturing plant, these raw materials are weighed and rigorously tested by analysts to determine their chemical composition and physical properties. If they pass all the tests, they are stored for later use.

Paragraph B

The raw materials are blended together with a proportion of recycled glass, known as cullet. Adding cullet to the batch helps to speed up the melting process. The batch is then fed through a regenerative furnace at a temperature of up to 1500°C, where it melts to form molten glass.

Paragraph C 

This molten glass is fed along a channel into the float chamber. The float chamber contains a bath of molten tin, which the glass floats on. Because tin has a higher melting point than glass, the glass can be floated onto the tin in a semi-molten state without the two substances mixing together.

Paragraph D

As the glass moves along the bath of molten tin, the temperature is gradually reduced from about 1000°C to 600°C. At this temperature, the glass is hard enough to be moved onto rollers without being marked, and the tin remains in liquid form. Because the surfaces of the tin bath are flat, the glass acquires a smooth, uniform surface, making it ideal for use in optical instruments. At the end of the tin bath, the glass has formed a continuous ribbon of flat glass.

Paragraph E

The glass ribbon then passes into a long furnace known as a lehr, where it undergoes a process called annealing. Annealing involves slowly cooling the glass in a controlled way to relieve any internal stresses that could cause cracking. The temperature is reduced step by step from about 600°C to room temperature.

Paragraph F 

As the glass emerges from the lehr, it is inspected thoroughly by cameras and electronic equipment to detect any defects or imperfections. Any faults that are detected mean that the section of glass must be removed from the production process.

Paragraph G 

After inspection, the glass moves along to the cutting section. A computer-controlled cutter scores the glass at the correct width. The glass is then snapped off cleanly along the score line and moves on for further processing.

Paragraph H 

The float process produces glass with a very smooth and flat surface without the need for grinding or polishing. This is because the glass floats on the perfectly level surface of the molten tin. The process has become the standard method for manufacturing flat glass used in windows, mirrors, car windscreens, and optical instruments around the world.

Questions 1–7 — True/False/Not Given

Do the following statements agree with the information given in the Reading Passage?

In boxes 1–7 on your answer sheet, write:

• TRUE — if the statement agrees with the information
• FALSE — if the statement contradicts the information
• NOT GIVEN — if there is no information on this

1.  1. Raw materials are tested before they are used in the glass-making process.
2.  2. Cullet is added to the batch to improve the quality of the finished glass.
3.  3. The price of cullet is lower than the price of the raw materials it replaces.
4.  4. The surfaces of the tin bath make it unnecessary to polish the glass afterwards.
5.  5. The annealing process takes place at a higher temperature than the tin bath stage.
6.  6. The cameras used in the inspection process were designed specifically for glass manufacture.
7.  7. The float process has replaced other processes for the manufacture of flat glass.

Questions 8–13 — Diagram Labelling

Label the diagram below.

Choose NO MORE THAN ONE WORD from the passage for each answer.

Write your answers in boxes 8–13 on your answer sheet.

The diagram shows the main stages of the float glass process.

1.    8.  Recycled glass = ………………… (added to raw materials)
2.    9.  The mixture of raw materials = …………………
3.  10. High-temperature heating unit = …………………
4.  11. Metal used in the bath = …………………
5.  12. Continuous flat sheet = …………………
6.  13. Device used for cutting = …………………