Neetu,

It depends on your maximum operating temperature. If the maximum temperature is 1300 C, I will suggest you to use pure platinum crucible. This is the best choice! However, in case you want to use the crucible at temperatures higher than 1300 C, you have two options:

1. Use pure platinum crucible with a ceramic liner. In other words, put a platinum crucible in an alumina or silica crucible and then melt the glass. This is because pre platinum becomes very soft at temperatures higher than 1300 C. Therefore, you need a ceramic liner (or jacket) to handle it at high temperatures.

2. Use a 90Pt-10Rh crucible. We do this in our lab all the time. I understand that rhodium can seep into the glass melts but this does not happen in appreciable quantities to affect your result. I have used Pt-Rh crucibles all my career and have never faced any problem. This is mainly a problem in industrial settings where they melt kilos of batch in big (or giant!) crucibles. You can use a 90Pt-10Rh crucible until 1625 C. We even use them until 1650 C (on regular basis) or up to 1675 C (rarely!), but I have got custom made thick crucibles to sustain these temperatures.

Regarding your question about reaction with alkalis, platinum hates alkali carbonates. Therefore, try to avoid alkali carbonates in your batch, especially if you are making a batch with more than one alkalis. Depending on the amount of alkali carbonate in your batch, it can even dig a big hole in the base of your crucible. In our lab, we prefer using alkali silicates or alkali borates as the source of alkali oxides.

Regarding 90-10 vs. 80-20 Pt-Rh crucibles, there isn’t much of a difference between the two in terms of their operating temperature. However, 80-20 is a bit stronger than 90-10. Therefore, 80-20 is generally used in applications where you cannot afford the crucible to be deformed. For example, in a high temperature rotation viscometer, the viscosity readings can be affected if there is a change in shape (dimensions) of the crucible. Therefore, you need a firm crucible in this case.

Another thing you need to know is that platinum does not like phosphorus or any reducing agent. Therefore, never ever melt a phosphate glass in Pt-Rh crucible. P2O5 in the amounts up to 20 – 25% are still okay, but not above that. Similarly, never use platinum in reducing environment. For example, in our lab, we melt glasses with sugar in them to create reducing environment. However, all this work is done in ceramic crucibles.

If ever melting a glass at temperatures 1700 C or higher in a platinum crucible, make sure you have a continuous supply of oxygen flowing through the furnace to maintain oxidizing environment.

I hope this helps. Feel free to contact me for any further questions.

Ashutosh

Dr. Goel

Thank you Sir, for clarifying my doubts. It would help me to decide the product that I need to buy. I am not very confident about the 1st point, to use platinum with a ceramic liner, as there remains a temperature gradient inside and outside the ceramic lining for a fraction of time, which has sometimes proved to be fatal for the platinum crucible.

I am considering 2nd point to be more safe, besides keeping the precautions with the composition to be melted. Thanks sir, for your valuable views and suggestions.

What should be the thickness in general for crucible, so that it can withstand pressure applied while holding with the tongs, at high temperature, during quenching?

Neetu

Again depends on the melting temperatures you are aiming at and the volume of melt you want to handle in the crucible. Please contact me at [email protected] for any further questions.