I have a couple of spagyrics that I wish to make, and with that process comes Calcination. It's how most people think of the start of the alchemical process, skipping the Putrefaction and Mortificatio that come ahead of it. That's one of the first lessons of lab alchemy versus a purely spiritual approach. It becomes clear that almost no process starts with fire, and that spiritual alchemy probably shouldn't either.
Recently, there was a post I read on Tumblr from an acquaintance, himself working on Calcination, and he was describing how difficult it was to achieve it. He had plant material in a closed crucible in his home oven, running as hot as it could for several hours, and did not have white ash. I sat down, went over a lot of my notes, looked in a few books and here's my post-Calcination interpretation of what is happening. Or more accurately what is not happening.
Mark also this with the most illumined of the Philosophers, that there is this difference, between vulgar calcination, effected by the force of fire, & natural calcination; that the first destroys the body, & consumes the greater part of its radical humidity; but that the second not only conserves the humidity of the body, it calcinates; but also considerably augments it.
from Letter to the True Disciples of Hermes by Alexandre Toussaint de Limojon, Sieur de Saint-Didier, 1688.
Calcination is where a material is burnt to the point of white ash. As much as a kitchen oven will clean itself by burning the layer of cooking grease on it's interior to ash, that does not mean a stove can burn anything else to white ash. In all honesty, it's not hot enough. The whole point of an oven is to provide controlled, precise heat so as to NOT burn what you are heating.
Really hot fires have access to lots of oxygen, and they have extreme temperatures. Let's look at campfires. They burn to white ash if made hot enough, but will leave blacken chunks of charred wood if not. Campfires burn between 700-1200 degrees depending on the wood used, the structure of the fire (how the wood is stacked) and turbulence (airflow over the burning material).
The first thing Robert Bartlett said to us about calcination when I took his Prima class was not to do it in our kitchens, and then he just started laughing. Apparently he gets asked about this all the time. Unless you have a wood-burning stove, and calcine inside that (which Robert sometimes does), a conventional cooking stove is not the tool to be using. Aside from a lack of heat, you also don't want all the crap that gets burnt off to get in your house or all over where you prepare food. You're doing chemistry, not baking a cake.
So, if kitchens are not the place to calcine, then what?
Good question. Calcination is best accomplished in the open air, with an open container, over open flame. Get outside and use your barbecue or a camp stove or a fire. Put your material to be calcined into a steel container. Have something with which to turn or stir the material as it cooks.
If you have plant material that has been in pure grain alcohol, you can light the alcohol and burn the material somewhat, but that's not enough fuel to consume the entire quantity at a level to produce white ash. That's another Calcination assumption beginning alchemists make that doesn't work for them.
With a kitchen stove, you can heat a material for five or six hours and just have black carbon. Over a camp stove, you can usually get to white ash in an hour because the area you are heating is focused and concentrated. It will always vary with what you are working with, and how large a quantity you are attempting to burn.
Take any and all fire precautions. Wear fire gloves when handling anything you have been heating, even if only for a little while.