puffing 

https://doi.org/10.1351/goldbook.P04941
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 [Terms] [Paper]