During the last quarter of the 19th C they started painting the mouth pieces with a special sort of wax, like a runnier form of sealing wax to stop the clay sticking to your lip and pulling a chunk off ( done it, hurts like crazy for the size of the wound ), before that they sometimes applied a slip/glaze before firing or polished the stem with wax or whatever. if you keep the mouthpiece wet ( dipping it in your drink or just wetting it in your mouth occasionally) you don't have a problem.
I've been picking them up for near 30 years when they have been cheap. Best buys was when a antique dealer emptied a fancy mahogany deep frame advertising display thing of pipes (Why!?) and he had the pipes in a flower pot for a couple of quid a go and I got all the longest ones in the picture, wish i could have bought the lot. Now a days you can pay the best part of 20 quid for a really long one, probably more now.
They did indeed make a sort of clay with a stronger stem of a different material, way back in the day you could just get just the bowl and fit it to a wooden , hollow reed or even iron stem. The best smoking short pipe I have is a clay with a plastic stem.
The bowls still fragile but not so bad as a traditional clay. I just have to remember not to tap it out on walls! I got it on a flea market stall for a couple of quid and it sat for a long time in its box unsmoked until earlier this year.
I've broken surprisingly few clays so far, you can usually find somewhere safe to carry them especially if you habitually wear a broad brimmed hat. Traditionally they were shipped in sawdust. My one attempt in trying to send one that way to someone resulted in it never arriving so i don't know if it worked or not!
Traditionally there was two ways of cleaning them, burning in a fire, usually in a sort of protective iron cage or burying them for however long ( none of the books have said and I've never tried it ). I assume something in the soil breaks down the gunk. No doubt plenty were never dug up hence the occasional finds of examples in still usable condition. Some where continued to be smoked when the stems were so short that they would burn the cheeks of the users, indeed some were made that short deliberately as they were easier to hold in the mouth on short stems, were less in the way while working and back when everyone wore a hat of some sort the closer to your face the more protection they got from the rain. Mind saying that smoking them upside down sorts the rain problem. Hardly owt falls out just you get even more funny looks than usual!
ATB
Tom