Resistance against thermal shock - Porcelain and ceramic bodies
The "technical" definition of a thermal shock can be found at the end of this article. The thermal shock is largely one of the natural enemies of porcelain and conventional ceramics and often appears invisible at first. By thermal shock we mean the rapid change from cold to warm (hot) or vice versa from hot to cold.
Where does thermal shock occur?
Especially in the gastronomy and canteen kitchen, the thermal shock is quickly caused by unskilled personnel. A typical scenario is: Out of the cold store and into the combi-steamer. In the hotel industry, bistros, cafés and in private households, the thermal shock always occurs when food, soups or hot drinks are transferred directly from the cooking pot - or from the machine - into non-preheated porcelain.
Prevent the thermal shock!
A good coffee machine does not have the cup warmer function on its top for fun! On the one hand, you can actually pour 70 - 90 °C hot water (tea, coffee, etc.) into pre-warmed cups, and on the other hand, the drink stays hotter for the guest longer (stable). Heating cabinets, plate warmers and heated plate dispensers do the same for flatware.
Any kind of preheating or preheating significantly reduces the temperature difference that exists in the event of a thermal shock. It is only a matter of reducing this temperature difference by a degree of reduction. Here is an example:
|Cup||Room temperature 21 °C||preheated hot water 38 °C||preheated dispenser 50 °C|
|Hot water for tea||92 °C||92 °C||92 °C|
|Thermal shock load||71 °C||54 °C||42 °C|
|Degree of reduction||0%||24%||60%|
Conclusion: If you preheat your dishes, you reduce the risk of a thermal shock considerably!
Thermal shock can be heard!
The breakage of the porcelain during a thermal shock can be heard clearly: There is a (1 x) soft crackling sound. In many cases, the fractures are not visible at first, which tempts the untrained user to continue using the respective part. This is a mistake! Once "cracked", the porcelain should definitely be taken out of use to avoid greater damage or even injury later on.
Thermal shock - an example of damage
Holst Porzellan supplies porcelain jugs for hospital food distribution systems. In a hospital, a patient reaches for such a jug and, in order to refill, guides it from the tray over his body. Suddenly the bottom of the jug tears and the patient scalds his thighs with the hot coffee. One day after the damage was reported, we immediately carried out a site visit.
It turned out: The coffee container on the station was incorrectly set or the thermostat was defective. The filling temperature was more than 80 °C and was therefore also above the drinkable temperature. The thermal shock load was 60 °C and more. An examination of other jugs showed that about 20% of all porcelain jugs showed traces of thermal shock.
The hospital had to pay the patient EUR 20.000,-- compensation for pain and suffering. Whether the manufacturer or the customer service of the coffee dispenser contributed to the damage is beyond our knowledge. If we had not been directly on the spot and had not secured the evidence, we would probably have been wrongly called to account!
The staff of this infirmary was not trained in porcelain and had not taken care of the issue. The damage clearly resulted from human error and lack of instruction.
Porcelain and ceramic tableware must be protected from thermal shock
In general, ordinary utility porcelain must not be exposed to thermal shock. The rule of thumb is: the thinner the shard of porcelain, the higher the sensitivity to thermal shock. Porcelain - also called hard porcelain (feldspar porcelain) - consists of two material components: Glaze and shards. The body of the porcelain reacts more slowly to thermal shock than the glaze. The more mass a body has, the lower is its proportional expansion coefficient. In other words: the thicker a porcelain body, the higher its resistance to thermal shock.
High Alumina by Holst Porzellan is resistant to thermal shocks!
All articles of the collection "High-Alumina" by Holst are resistant to thermal shock!
Due to its high density and special formula, High Alumina porcelain is one of the few porcelain tableware that is resistant to thermal shock. High-alumina porcelain can withstand a thermal shock of up to 200 °C. However, in order to compensate for production-related fluctuations and to maintain a "speed reserve", we guarantee thermal shock resistance of +/- 180 °C, for example from minus 20 °C to plus 160 °C.
This makes High Alumina porcelain particularly suitable for use in convectomats to regenerate food, for convenience food and for baking or re-cooking directly in the oven.
Thermal shock is the rapid, shock-like change in the temperature of a material. This leads to mechanical tensions between the outer (jacket) and the inner area (core), since a material heats or cools down faster on the outside than on the inside. The material is permanently damaged if the temperature difference exceeds a certain value. This critical temperature value is influenced as follows:
1. The linear thermal expansion coefficient a [K -1] indicates the percentage elongation of the material at a temperature change of 1K. A material with a small coefficient of linear thermal expansion is more resistant to thermal shock.
2. The thermal conductivity l [Wm -1 K -1] indicates the speed of heat transport through a material. A high thermal conductivity provides greater resistance to thermal shock.
3. The Poisson's ratio n is the ratio between longitudinal expansion and transverse contraction. A smaller Poisson's ratio also provides a higher thermal shock tolerance.
4. The modulus of elasticity E [GPa] is a measure of stiffness. A lower modulus of elasticity results in a higher thermal shock tolerance.
5. The strength sf [MPa]. A higher strength increases the tolerance to thermal shock. After the thermal shock of the samples, the modulus of elasticity should be measured by means of an acoustic method. The vibration of the sample is determined by means of a small hammer and a microphone. Furthermore, the residual strength after the thermal shock shall be determined by a 4-point bending test. The strength is the quantity that indicates the force that must act on a sample to cause it to break. This is the strength that the sample still has after the thermal shock.