Earlier than pour tea into cups, experienced housewife, taking care of their safe, don't forget to put them in a spoon, especially if they are silver. Practical experience has developed quite correct reception. What is its basis?
You have to understand first why the glasses are cracked from hot water.
The reason is the uneven expansion of the glass. Hot water, poured into a glass, heats its walls did not immediately: first heats the inner layer of the walls, while the outer has no time even to feel warm. Heated inner layer immediately expands, the outer also remains unchanged and has, therefore, a strong pressure from the inside. There is a break - glass bursts.
Don't think you will secure yourself from such “surprises”if you have thick glasses. Thick glasses are the most fragile in this respect: they burst more frequently than thin. It is clear: the thin wall warms up faster, it quickly established a uniform temperature and uniform expansion, as in thick, slowly warmed the glass layer.
Just about one thing we should not forget, choosing thin glass dish: thin should be not only the side walls, and the bottom of the glass. When pouring hot water is heated mainly the bottom; if it is thick, the glass will crack, as if a thin nor were its walls. Easily burst the glasses and porcelain cups with a thick circle ledge at the bottom.
Than glass vessel thinner, the more confident you can expose it to heat. Chemists use very thin vessels and boil them in water directly on the burner, not anxious for the integrity of the vessel.
Of course, the ideal crockery would be such that it is not expanded when heated. Very few expands quartz: 15 - 20 times less than glass. Thick vessel made of transparent quartz can be arbitrarily heated - it will not burst. You can safely throw a quartz vessel, heated to red heat, in cold water, not fearing for his integrity [Quartz ware convenient for laboratory use because it is very refractory: quartz softens only at 1700°]. This is due in part to the fact that the thermal conductivity of quartz is significantly greater than that of glass.
Glasses break not only with the rapid heating, but with a sudden cooling. The reason is uneven compression: the outer layer, cooled, contracted and strongly compressing the inner layer, has not yet had time to cool and shrink. Therefore it is not necessary, for example, a jar of hot jam to put on the sharp cold, immerse in cold water, etc.
But let us return to a tea spoon in a glass. What is the basis of its preservative action?
The dramatic difference in heating of the inner and outer layer of the walls is only when the glass is immediately filled with very hot water; warm water does not cause a sharp difference in the heat, and consequently the tension of the various pieces of glass. From the warm waters of the ware does not tear. What happens if the glass is put the spoon? Once on the bottom, hot liquid before heating the glass (which is a bad conductor of heat), manages to give part of their good heat conductor - metal; liquid temperature decreases; from hot she is warm and therefore almost harmless. The further prelivanje hot tea is not so dangerous for the glass, as he had a little warm up.
In short, metal spoon in the glass (especially if it is massive) smoothes the uneven heating of the glass and thus prevents cracking of the glass.
But why, if silver spoon? Because silver is a good conductor of heat; silver spoon quickly takes heat from the water than copper. Remember how silver spoon in a glass of hot tea burn my hands! On this basis, you can even accurately determine the material of the spoon: copper spoon fingers does not burn.
The uneven expansion of the glass walls jeopardizes the integrity of not only the tea cups, but critical parts of a steam boiler is its water pipes, which are used to determine the height of water in the boiler. The inner layers of these glass tubes heated by hot steam and water, expands more than the outer. The tension arising from this cause, add another strong pressure steam and water in the tube, making it easy to burst. To prevent this, sometimes produce water tube of two layers of glass of different varieties: the inner layer has a lower coefficient of expansion than the outer.