1 Clay comes in varying degrees of porosity and some will absorb a lot of flavours from tobacco.
2 Putting it in a fire is not the same thing as firing it in a kiln and there is a risk of making the clay more brittle.
3 One thing to be aware of is that contrary to what some people think, clay pipes do need breaking in as when new they can exhibit a noticeable ceramic taste.
4 Remember though that a clay will present tobaccos very differently from briars or cobs and you may dislike a blend smoked in a clay that you would otherwise enjoy so be sure to try it in a variety of pipes before dismissing it.
I agree and disagree on some of these points. I respect your opinions in where we disagree and I only bring up my point as a counter not as an authority on clay.
1. I do agree that clay depending on the maker and clay involved could absorb oils from the tobacco. I have not personally seen this but I'm going to go out on a limb and make a educated guess as to how that could happen. A low grade dig it up in your backyard clay could very well be bad for ghosting. This is one of those times to avoid ghosting knowing your pipe maker and what type of clay they use can be helpful. They're cheap enough though that if you find a particular brand ghosting it can be replaced inexpensively if this is a problem.
2. I absolutely agree a re-fire can definitely lead to a brittle pipe... it's already brittle... how I get around having to do this is by using a small baby tooth brush for the inside of the bowl and soft cloth for the outside and a spirit of some sort. I use straight whiskey personally as it imparts the least amount of "flavor" to the inside of the pipe.
3. I disagree with the breakin. I wash my clay pipes before smoking them. Why? Because every once in a while the manufacture leaves clay dust on the pipe inside the stem and on the bowl. Once washed and left to dry (several days to avoid any chance of cracking under heat, maybe not necessary but it's part of the ritual for a new clay pipe for me personally.) I've never tasted a ceramic flavor from the pipe from the first bowl onward. Secondly I would like to point out most tobacconists when blending tobaccos use clay pipes to test their blends with as the clay will not impart a flavor to the tobacco. I've heard of a few using meerschaum but as they are leaps and bounds more expensive it's more to show off than anything.
4. I totally agree! brier and cobs impart a flavor so it's definitely worth trying on several different pipe types.
