Hi Cornish Jack,
The absence of a substance from the DGR book does not mean that the substance cannot go by air. Likewise, its presence in the DGR book does not give it the right of passage.... look at all the forbidden substances in Section 2 and in Section 4 (Blue Pages). New substances and materials are coming on the market all the time and many of them are dangerous.
On many occasions I have been presented with the argument that if a substance is not listed in the DGR, it is OK to travel by air... Balderdash! This usually happened after I discovered some "know-it-all" pratt loading DG into an aircraft.
The shipper must decide the nature of his/her substance/material using guidelines in the DGR and in the ICAO Technical Instructions fo the Safe Transport of Dangerous Goods by Air (the legal source document for the IATA DGR).
Having established the Flash Point and other characteristics of, for example, a flammable liquid, the shipper then establishes the Packing Group, thereby indicating the degree of danger. Provided all the requirements are met (packaging, documentation, labelling, etc...too detailed and numerous to mention here, the shipper can give the substance its Proper Shipping Name of, for example, Flammable Liquid N.O.S.
To recap, the presence in or absence from the DGR book of a specific substance/material does NOT necessarily mean that it can or cannot travel by air. It's just not that simple. The current DGR book probably went to print some time last autumn and at that point in time, it was reasonably up to date. How many new concoctions have emerged since?
Get on a full DG training course and all will be explained!
And yes you are right to treat "Dangerous when Wet" with the greatest of respect.
I hope this helps.