@Z100000000000m
You asked why Pearlash was made at a clay oven (which is called a kiln), I pointed out that pearl ash is actually made in a kiln in real life. I also pointed out that Toady, the developer of DF, put the reaction there and there is no reason for Meph to change it.
You continually ask why something isn't in the manual, without the understanding that MW is a mod to the vanilla game. If the MW manual doesn't have the info, then the next source to go to would be the DF Wiki because what you are looking for is part of vanilla. And if you knew that inside and out, then you wouldn't be asking about pearlash.
For example: What's the melting point of copper? : well, copper is a vanilla metal, so check the wiki.
What's the melting point of welded mithril? : well, welded mithril is a MW metal and the information is not in the included manual, so ask on the forums.