puffing

An irreversible expansion of some carbon artifacts during graphitization heat treatment between $1650$ and $2700 K$ .

Note:

Puffing is caused by the release of heteroatoms, for instance sulfur atoms, from the coke in association with specific microstructural rearrangements.

Source:

PAC, 1995, 67, 473 (Recommended terminology for the description of carbon as a solid (IUPAC Recommendations 1995)) on page 502