Your question is surprising. Of course, as colleagues said, a too high temperature may decompose the clay by production of water vapor. Therefore, you material will be modified by the degassing process, and the surface area that you'll measure afterwards will not be representative of your material. Hence the need of decreasing the heat-treatment temperature.

But you talk about "charring", which means that you material turned black by carbonisation !! Where this carbon comes from ? The purity of your clay is questionnable since you should not observe formation of carbon.

Before you go further, you should think about purifying your material. If heating in air makes it turn white again, than it proves that you have burnt the carbon. It will confirm that your clay was full of organic impurities.

Alain

Dear Manasa, The material will has hydroxy groups in it. These group dissociate to form water at elevated temperature. Hence you are observing the material as charred. I would suggest you to perform BET degassing at lower temperature. To avoid dissociation of hydroxy group from the material.

You probably need a lower temperature for your degassing. It is preferable to run a TGA of your sample prior to degassing, to see at which temperatures your sample will degrade. Even if your sample's first degradation is above 300 C, it may degrade at a lower T due to being under vacuum; for example, I have recently degassed a MOF sample at 180 C, knowing that its first degradation in air was at 250 C, however it still degraded at this T under vacuum.

If you are unsure of the degradation profile of your sample, I would use a low T for degassing (~90C), and use a long degassing time (~32 h) to ensure a full degas.

Obviously, any results you get from BET on these charred samples will not be relevant to the original clay material.

Your question is surprising. Of course, as colleagues said, a too high temperature may decompose the clay by production of water vapor. Therefore, you material will be modified by the degassing process, and the surface area that you'll measure afterwards will not be representative of your material. Hence the need of decreasing the heat-treatment temperature.

But you talk about "charring", which means that you material turned black by carbonisation !! Where this carbon comes from ? The purity of your clay is questionnable since you should not observe formation of carbon.

Before you go further, you should think about purifying your material. If heating in air makes it turn white again, than it proves that you have burnt the carbon. It will confirm that your clay was full of organic impurities.

Alain

I want to believe that your sample may contain some organic impurities or bio polymers. For most clay materials, the dissociation of its hydroxyl groups do not occur until above 400C. I work with a couple of clay materials and carry out BET degassing at 200C for about 24h. I will suggest that you purify your samples before analysis.