Glass is a high viscosity liquid, amorphous, rigid, transparent and homogeneous, obtained through the slow cooling of molten silicates. The main component of the flat glass and the silica sand (SiO2), whose melting point is around 1700 ° C.
From silicon to vitreous silica
La crystal structure of silicon has the shape of a tetrahedron: in the center there is a silicon atom, connected symmetrically to the four vertices of oxygen atoms, the chemical formula is in fact SiO4. Rapidly cooling the fused silica produces one random structure of tetrahedra, joined together at the corners, which gives rise to an amorphous material, vitreous silica.
Discovering raw materials
The raw materials used in the manufacture of glass are divided into four groups:
- vitrifying. By the action of heat, they pass from the crystalline to the amorphous form. These substances retain their amorphous state at all temperatures; in fact, the viscosity of the fluid blocks the mobility of the molecules and makes crystallization impossible.
- Dark. They facilitate the melting of vitrifying substances at temperatures obtainable in industrial furnaces; they allow the subsequent processing of the glass and introduce alkaline bases in the acid components of the vitrifying agents.
- Stabilizers. They make the material unalterable and resistant to atmospheric agents.
- Accessory. Depending on their functions, they are divided in turn into: refining, bleaching, opacifying and coloring agents.
Chemical inertia
Glass is a material with excellent chemical inertness towards almost all acid and neutral substances. It also has good resistance even in the presence of weakly alkaline pH. These properties, guaranteed by the stability of its amorphous silicate network, make our protagonist the best material to hold food, beverages, pharmaceutical preparations and chemicals or reagents.
Strength tests
To verify the resistance of a glass composition to chemical aggression, various tests must be carried out, both following standardized methods and through customized tests, according to the needs of the customers.
Let's find out what it is, guided by the explanations of the Stazione Sperimentale del Vetro.
Resistance to alkali according to ISO 695
The test is carried out by suspending, by means of an inert wire, a planar glass sample of standard dimensions inside an aqueous alkaline solution, which is brought to a boil inside a special sealed metal container. At the end of the test, the degree of alkaline corrosion to glass damage is estimated gravimetrically, i.e. by measuring the weight loss suffered by the sample as a result of the test.
Resistance to acids according to DIN 12116
The starting conditions are the same as in the previous test, but an aqueous solution of hydrochloric acid (HCl) is used here, which is brought to a boil inside a special glass container under reflux conditions. At the end of the test, the degree of acid corrosion is estimated gravimetrically, i.e. by measuring the weight loss suffered by the sample following the test.
Hydraulic resistance of powdered glass according to ISO 719 and ISO 720
The test is carried out by subjecting some grain vitrea, having a well-defined particle size, to the attack of milliQ water at 98 ° C (ISO 719) or 121 ° C (ISO 720). The amount of alkaline ions released from the glass during the test is then determined by titrating the test solution with HCl: the higher the original alkali release, the worse the chemical resistance of the glass to hydrolytic aggression.
Resistance to washing in the dishwasher
Glass tableware must guarantee an adequate level of durability against aggression by the detergents used to clean them in the dishwasher.
Using a special dishwasher, the various items can be subjected to standard washing cycles, provided for by sector regulations, to determine their resistance.
Sources: spevetro.it, pilkington.com, chimicamo.org
