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What are the Functions and uses of Crucible Tongs and their Types

Its design (bottom image) is very reminiscent of crab claws, which close firmly on their food.

The same happens with the crucible forceps. Some of them even have a mechanism that secures or isolates the end that holds the crucible, making handling safer in principle. These clamps find use not only in analytical laboratories but also in the metallurgical industry.

In general terms, they are very useful when you want to calcine a sample, placing it inside the same crucible in a flask; or when metals are melted and the incandescent liquid is then transferred to other sections.

Crucible Tongs
Regular size crucible clamp. Source: GOKLuLe / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Crucible Tongs Functions

Crucible tongs. Source: Lilly_M / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Drawing of Crucible Tongs

Crucible tongs look at first glance as if scissors and pliers have been mixed together. Its size can range between 30 and 45 centimeters. The small versions are manipulated directly with one hand, while the large ones require that they be worked with using both hands.

The forceps hold a ceramic crucible possibly used for calcination. To be able to hold it, the clamp has rounded and curved ends, in such a way that it has a concave hole where the crucible or any other object with a round surface fits perfectly.

Many tweezers usually have their fingers or the grip covered with aluminum foil, in order not to scratch the surface of the crucibles or the glass material that is being held or held.

Physical Uses

Crucible tongs must necessarily be resistant to high temperatures (above 500 ºC) and corrosion. It is for this reason that most of them are made with stainless steel, although there are also other types of steel. We also have zircon, brass, nickel, and platinum clamps.

They are generally not very heavy and a single hand is sufficient to handle most of the time. However, there are more robust designs that require two people to hold a much larger container.

The fingers or the grip of the crucible tongs are practically supplanted or substitute for our fingers when it comes to holding hot or corrosive objects. Using it, you avoid putting your hands inside the fired muffle, in addition to removing the red-hot crucible from it with other less efficient and safe resources.

For many analysts, it is more comfortable to manipulate the forceps with one hand, so that one side of the crucible is grasped as shown in the image below:

The advantage of using the forceps with one hand is that the other is free to open and close the flask, without the need to leave its door open, radiating harmful heat to the surroundings. However, there is a risk of contaminating the content of the crucible, as well as of turning it over when depositing it on a thermal insulator.

With a little more dexterity, and using both hands, the contents of the crucible can be poured into other containers. This applies especially to molten metals, whose incandescent liquid will fill molds that give them all kinds of shapes.

Security measures

The reason these clamps have elongated designs is to keep the crucible away from the analyst or worker as much as possible. However, this alone is not enough to ensure complete safety during the process. Therefore, other security measures are recommended, which are:

  • Wear leather gloves, to prevent any type of splash from falling on the hands
  • Wear safety glasses
  • Make sure the crucible is dry so that water vapor does not originate during heating
  • Also, make sure that the tweezers are dry when holding the crucible
  • Do not fill the crucible to more than 2/3 of its content, to avoid the risk of splashing
  • Clean the tweezers constantly to remove any remaining grease
  • Do not use the tweezers for electrical purposes
  • Walk cautiously when holding the crucible

Since the contents of the crucible are still very hot after being removed from the muffle or furnace, it is essential to follow these and other measures as well as possible. The same applies if said content is extremely corrosive enough to risk transferring it even with gloved hands.

Usage examples

Crucible tongs taking oysters from a tank. Source: Oregon State University / CC BY-SA (https://creativecommons.org/licenses/by-sa/2.0)

Calcination

In laboratories, the use of crucible clamps is quite frequent in analyzes that require the calcination of a sample, either to determine its moisture or ash content.

The sample is weighed into the crucible, placed in the flask using the clamp, and with it is withdrawn to put the crucible to cool, to finally transfer it to a desiccator.

On the other hand, the clamp is also useful when combustion or thermal decomposition reactions are carried out.

Foundry

In the glass and metallurgical industry, crucible tongs are used to melt metals and glass respectively. When they melt, they become incandescent liquids, which are much hotter than samples calcined in laboratories. For this reason, tweezers are even more appreciated in these spaces.

Handling of corrosive substances

In cold conditions, crucible tongs are used to handle vessels containing highly corrosive substances or mixtures, such as strong acids or bases. Generally, this applies when working with a considerable volume of corrosive liquid, where the risk of splashing is greater.

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