Gray rP in my experience is a sign of oxidized rP. Every time I ruined some with undiluted H2SO4, I had gray, clay-like crap instead of rP. I take that to be oxidized rP.
Burn tests will show you whether you have rP as opposed to something else that looks like it. I've never seen the red glass and trash left after multiple reactions of mbrP burn with the same vigor as fresh red. If the ratio of rP to glass and trash is low, you will not have the vigorous, smoky burn that higher concentrations of rP will give you. The flame that ignites rP, however, may also be what burns away a coating of wax that prevents the rP from reacting with I2 in the flask unless cleaned. A burn test may help determine if the stuff is rP; it does not per se indicate it will work in the reaction. The rP in mbrP gets used up in the reaction, but the glass does not, and over a series of reactions the glass becomes more of the weight and throws the novice off, as he has too little actual rP in the glass/rP mix. The glass does not hurt the reaction at all, so long as sufficient rP is in that mix to drive the reaction. The difficulty with reusing mbrp is determining just how much of it is glass and how much is usable red P. Five uses, even with refreshing it with fresh mbrP, is about the limit of safe use on mbrP in my experience, so I side with Ware on that point.
Lab grade red phospohorous is consumed in a long wet reflux until it is used up. I have used lgrP with no more than alcohol and acetone cleanings between reactions until it has been consumed. I have done "last reactions" with rP as my supply dwindles before acquiring more with a ratio as low as 0.4:1 with the pseudo, or 0.33:1 pseudo to I2, and had success with a 48 hour reflux. This ratio leaves no margin for error. These reactions usually leave a very little bit of rubbery reddish residue that does not act like red phosphourous anymore. I have also done reactions at that ratio with fresh lgrP and had them complete successfully, leaving the same small bit of reddish, rubbery residue. If I have additional red phos I will add a little more to the reaction to assure myself I have enough, but I have reduced the amount of red P in my wet refluxes to nearer the molar amount needed without ill effects.
