This is one of the most useful collections of generic rules you are likely to encounter on the effects of immersion into a hostile natural environment (in this case, lava). The rules succinctly cover partial and complete immersion, and implicitly consider whether the immersion was accidental or voluntary (and if the latter, whether the decision was made by the immersed Character or by some third party who pushed the Character into the lava).
True, there is some ambiguity in the phrasing of the rules themselves (something which can hardly be surprising when one tries to write rules that can be applied to almost any game system). But that’s where the diagrams, brilliant in their ability to synthesise the key points of the rules, come in.
If I could point out a minor flaw or three, they would constitute my only criticisms of an otherwise brilliant book.
1) The decision-making flowchart (the second diagram) seems to create an unintentional loop. I would have redirected the rightmost “No” branch to another, new rhombus, perhaps reading something like "Do you want/have to remain where you are?”. Obviously, a “Yes” arrow would lead back to the middle rhombus, thus completing the loop, but this would then be the Player’s choice rather than the GM’s. On the other hand, a “No” response would lead to the yellow square (which might require minor rewording to consider this new possibility).
2) I have some questions concerning the special section on Characters who are “Immune to Fire”. Specifically, I should like to know what happens if the Character is immune to FIRE but NOT to HEAT? Conversely, what happens if the Character is immune to HEAT but NOT to FIRE? And perhaps the most insidious dilemma: what happens if the Character is immune to BOTH heat AND fire, but CAN’T SWIM? I’m looking forward to a second edition of these rules, which will hopefully address these pressing questions in sufficient detail.
3) One obvious formatting error is the total lack of numbering for the various illustrations, which might result in some confusion for the hasty reader. But a more leisurely perusal of the section, along with numerous references to the rules themselves, will likely clear that up in no time.
On the other hand, an obvious bonus is the inclusion of a very complete index. This, along with the care with which the authors worded the Disclaimer, shows their dedication to furthering the cause of good, clean gaming. If they're ever in my neck of the desert, I will proudly stand them a beer.
All in all, and despite minor flaws, this is possibly the best compendium of rules on Lava, Magma, and Superheated Rock ever written. And not only is it not rules-dependent, which is a great plus, it is also more polyfacetic than the authors themselves seem to realize. In fact, I will confess I tried something truly far-fetched, and it worked: I was able to adapt these rules – with minimum tweaking, mind you – to miniature wargames, and they have proven to be a great success indeed. (That last point, by the way, is the reason why, despite the aforementioned minor flaws, this book deserves a full five-star rating.)
In short, I heartily recommend this book. It’s free, true, but don’t let that deter you: it’s easily worth twice the price.