Thermo-gravimetrc analysis, often refered to as TGA, measures the mass of an unknown **solid** substance as it heats up, of course for the elemental analysis of the very substance. When the substance is heating up, due to special chemical or physical reactions, the mass is decreasing. e.g.: Dehydration. Anyhow, we know that in special circumstances the mass of the substance will increase. When? *That* is the question.
( This is not quantum physics! I am talking about rest mass, if you had wondered. As a result, the increase will have nothing to do with Einstein's relativity formulas.) The diagram below is an example of some normal polymer tested in normal conditions, no relations with the exceptions! 

![](https://i.stack.imgur.com/Xwo8F.png)
![enter image description here](https://i.stack.imgur.com/CBnbs.png) 

update: There also is a special type of gas that is blown above the surface of the substance. The system is designed in a way that doesn't allow the gas to "interfere" with the measurements. And I just got lucky to find another substance experimented. I estimate it is going to be a liquid alkanol, maybe ethanol. Diagram two displays the experiment's results.
I know it seems a little bit vague, but all the info that I provided at above is all we have got to answer the question.