What is the dirt that cannot be wiped off the glass?

When it comes to glass, everyone may be familiar with it. Whether it's their own windows or the windows of the building across from them, they are basically made of glass. This is not only because of its excellent optical properties (light transmission), but also because it has excellent physical protection properties (wind and rain resistance). It is also very affordable in terms of cost and materials, and its presence can be found everywhere.

The earliest glass in the world was made by the ancient Egyptians. The emergence and use of glass have a history of over four thousand years. In China, people found glass tubes, glass beads and other items in the tombs of the Western Zhou Dynasty, which indicates that the ancients began to manufacture and use glass in the Western Zhou Dynasty. Before the Northern and Southern Dynasties, people commonly referred to transparent objects made by burning as "glass"; During the Song Dynasty, it was referred to as "glass"; During the Ming and Qing dynasties, it was customary to refer to materials that were fired at low temperatures, semi transparent, and had a glassy luster as "glass".

Eastern Jin Dynasty glass jars unearthed from the Gaosong family tomb in Xianhe Temple, Nanjing

The term 'glass' is also mentioned in classical Chinese works. For example, in the poem "Children Playing with Ice" by the Southern Song Dynasty poet Yang Wanli (1127-1206):

In fact, according to current standards, many of the "glass" referred to at that time did not necessarily mean the "glass" referred to in modern times.

Nowadays, glass has become an important material in daily life, production, and scientific and technological fields. Glass can reflect and refract light, so it can be used to make lenses, prisms, or optical fibers for high-speed transmission; If metal salts are added to glass, its color will change, so colorful artworks can be made from glass, such as the famous stained glass window.

The most common type of glass is sodium calcium glass, which is mainly composed of silicon dioxide (SiO2), sodium oxide (Na2O), calcium oxide (CaO), and other additives. Glass exhibits chemical inertness in daily environments and is generally insoluble in acids (exceptions: hydrofluoric acid (HF) reacts with glass to form SiF4, which can cause corrosion of the glass); But soluble in strong bases such as sodium hydroxide (NaOH).

Common glass corrosion phenomena

In people's perception, glass in daily life may be considered a very stable substance that is not easily corroded. If you carefully observe the mirror in the bedroom or the glass on the window, you will find that with prolonged use, white mist and white spots will appear on the surface of the glass, which cannot be removed even after wiping with water. These white mist and white spots are not dust, but products of glass corrosion. Some people also use "glass weathering" and "glass mold" to describe this corrosion phenomenon.

(a) White mist and white spots on the glass of the bus; (b) Rainbow Spot

At the turn of spring and summer, as the temperature and humidity gradually increase, the likelihood of glass corrosion greatly increases. For building window glass that is only subjected to atmospheric erosion, its corrosion can be divided into four stages:

(1) Substances such as water, carbon dioxide, and dust in the air can adsorb onto glass surfaces to form deposits, but these surface deposits are easily cleaned with water.

(2) If the glass is kept under high humidity conditions (above 60%), the surface sediment will become thicker (about 1 μ m), and water or moisture will diffuse into the glass. The hydrolysis of glass components will cause the surface to change color, but it can still be cleaned with water.

(3) After the formation of a deposition film on the surface of glass, if it continues to be in a high humidity atmospheric environment, certain components (alkali metals and alkaline earth metals) will accumulate and leach on the surface of the glass, and the soluble silicates on the surface will be destroyed. At this time, there will be substances with high silicon content in local areas, which look like water stains. The slightly corroded parts can be cleaned with vinegar, and even the severely corroded parts need to be effectively cleaned with hydrofluoric acid as mentioned earlier.

(4) In the final stage, the glass surface will become very rough and severe corrosion will occur locally. This is because sodium ions in the glass migrate out of the matrix and react with air to produce white alkaline substances, which are white mist and white spots on the glass. Even using hydrofluoric acid cannot achieve effective cleaning.

Perhaps people may not fully understand the roughness of glass surfaces. For example, in daily life, the longer a mirror is used, the more it becomes blurry. This is because the surface has been corroded, becoming rougher and causing diffuse reflection of the incident light. Considering the first stage of glass corrosion, gently wiping the mirror with water every week to keep it clear and clean can extend its service life.

In addition to white mist and white spots, glass corrosion can also cause rainbow spots, which have complex causes and can be divided into two types:

(1) The stress spots generated during the production process are also known as stress spots. There is currently no way to avoid the phenomenon of incident light interference caused by uneven stress distribution inside glass.

(2) Corrosion caused by water vapor or other chemical products in the air during use.

Corrosion of glass has both drawbacks and benefits

Although glass corrosion can bring troubles to our lives, it is a useful processing method in the glass processing process. Someone may be curious about how text or patterns are added to glass?

In addition to direct carving, etching agents (HF, hydrofluoric acid) can also be used to etch the glass. Coat the glass with paraffin solution dissolved in gasoline as a protective layer, and carve the required text or pattern on the cured paraffin layer, with a depth that allows the glass to be exposed; Then, drop hydrofluoric acid onto the text or pattern, control the corrosion time, adjust the depth of the desired pattern, and then wash away the paraffin and hydrofluoric acid with warm water to produce patterned glass. (Not recommended for self operation, hydrofluoric acid has strong corrosiveness)

Using HF to etch glass

The chemical reaction equation is: