Don't smoke pipe anymore, reason, the tobacco burns my tongue!!
One of the great unsolved problems.
I know quite a few people who prefer a cigar over the pipe for that reason (although they won´t admit it...). And the main difference between cigar and pipe is that a cigar provides natural filtering through its length. Actually, my own search for the right pipes and tobaccos was mainly led by the idea that a pipe should be smokable as slow and relaxed as a good cigar.
The standard advise from experienced pipe smokers is that you just need "good smoking technique". But that will only take you so far.
Here´s what I´ve learned:
Tongue burn is different from tongue bite. The temperature of the smoke stream near the mouthpiece is normally not very high, even when the bowl is so hot that it can´t be held comfortably anymore (I never felt an ebonite mouthpiece was getting soft in these instances, while hot water can be used to bend such a stem). Fast deep draws during initial lighting may indeed cause some burn, so just being more careful here can already solve the problem.
Tongue bite seems to be of chemical nature and an entirely individual matter. Some people cope better with sweeter and more acidic tobaccos, others better with alkaline ones. The same for additives. Basically all commercial pipe tobaccos regarded as "natural" contain additives like invert sugar, flavours and PG, too, and are therefore not a priori better than balanced "aromatics". The only way to find out what agrees with you personally is to test. I have sampled many highly praised tobaccos and often enough they simply didn´t agree with me. I routinely add my own Oriental and/or cigar leaf to commercial mixtures now, which smoothens things out.
In my observation, even experienced pipe smokers may feel tongue bite again after not smoking for a while. This suggests that similar to eating very spicy food there´s a certain component of "being used to it".
The mouthpiece: One of my favourite pipes was distinctly more prone to bite than the others. From the outside, the mouthpiece had the same straight slot. But on closer inspection it was actually very shallow, and the smoke stream came sharply out of the unopened bore. I could actually feel the stream hitting the tip of the tongue in a very concentrated manner.
Re-shaping the slot to a deep V-shape made a huge difference. (Pipe makers seem to spend a lot of time on that in more expensive models). The diffused smoke feels much smoother. (Note the similarity to cigars.)
Dry tobacco with good burning properties helps a lot. If you have to draw continuously just to keep the pipe running something is wrong. I prefer tobaccos which allow for half a minute to a minute between draws, and still come back. You may have to experiment with different cuts and packing techniques here (flake tobacco cut into cubes usually works well for me, but my Orientals don´t need more treatment than an ordinary ribbon cut).
Aged tobacco helps a lot, when most of the more aggressive components have mellowed. Some cigars can bite, and I think that´s for the same reason.
An absorbent pipe helps. Light briar, porours meerschaum or corncobs seem to be less troublesome. Changing the pipe often (and letting it dry out properly) can make a difference if you experience tongue bite only after 2 or 3 bowls.
Long pipes help somewhat if one resists the temptation of drawing harder, I guess some of the heavier components of the smoke condensate on the way through the stem before reaching the mouth.
Metal condensation filters in pipes don´t do anything for me except perhaps keeping small tobacco pieces from traveling up.
Paper filters can work well against condensation, Balsa filters too, the 9mm Savinelli´s seem to absorb as well a bit of nicotine. I found they help somewhat against burn, not necessarily against bite. Paper filters should be removed after each bowl, the Balsa filters can be left in for a second bowl, if smoked soon after. Meerschaum filters or Meerschaum granulate in the bowl are similarly effective, while perhaps tasting a bit chalky.
What makes the biggest difference for me (after following all of my own advice above) are activated charcoal filters. They are (like aromatics) despised among "serious" pipe smokers, but I do not feel they alter the flavour to the worse, and I never had an instance of tongue bite anymore. I only know Stanwell 9mm filters first hand, but there are others with (supposedly) a bit more or less filtering effect. An interesting side effect is that the pipes are totally dry after smoking, smell much better and the stems stay clean.
The charcoal filters do decrease nicotine quite a bit, which I welcome, since it allows me the enjoyment of tobaccos I had to be careful with before.