"Stoving" is a somewhat vague term that always refers to heating the tobacco. The divergence is that "stoving" sometimes refers to baking or toasting (under a confusing range of temperatures and durations), but may sometimes refer to applying moist heat (e.g. steaming). Higher temps usually result in darker tobacco: darker red, brown or even black. Steaming darkens tobacco to brown and eventually to black. Nearly all of the "stoving" techniques used by commercial pipe tobacco manufacturers are closely held proprietary secrets.
Some commercial stoving methods continually toss the shredded leaf within a rotating drum that is heated. (I consider this toasting. This is how cigarette manufacturers "toast" their burley.) Others use a "steam jacket" for the heat--a description that may mean steam-heated press plates, or may mean a mechanism for actually delivering the steam to the leaf.
The common thread is that both dry heat as well as moist heat will alter the color and flavor of the tobacco in ways that invite experimentation.
Some purchasers of tinned tobacco "stove" it in a hot car, or even in a Crockpot, with wildly varying results, depending on the tobacco blend, the temperature reached and the duration. The outcome can be wonderful or horrible.
One approach to stoving leaf at home is to pack it into a canning jar (e.g. Mason jar) and seal the lid. This can then be placed into a pot (or Crockpot) of water that is heated to a specific temperature (like sous vide cooking) for a carefully measured duration. Or you can cook the jar at boiling (or higher, in a pressure cooker), until you get the color desired. A variation of this method is the spectrum of initial leaf moisture, from nearly dry to soggy.
You can also just pile leaf into a colander that is placed over a pot of boiling water. (I regard this as essentially a Cavendish method.) Turning it completely black takes about 8 hours of steaming. Done similarly in a pressure cooker (i.e. leaf exposed to the contained steam) takes about 5 hours to blacken tobacco.
Some folks place shredded tobacco onto a cookie sheet, and toast it in the oven, periodically taking it out and misting it with water (or casing).
I have placed beautiful lemon Virginia flue-cured tobacco into a kiln at ~125ºF and RH ~75% for a month. The leaf came out redder, more flavorful and less acidic than the original lemon Virginia. I don't consider this better, per se, but rather different--yet another blending component.
The usual flue-curing process destroys (at ~149ºF) the most active cellular oxidase enzyme (responsible for "aging"), but not the less active cellular peroxidase (~191ºF). So flue-cured tobacco (typical max temp of 165ºF), if left alone, does "age", though fairly slowly, when compared to leaf that has never been taken to a temp as high as 149ºF.
Although many popular commentators (e.g. GL Pease and Russ Oulette) attribute aging to microbial processes, I believe most changes of aging are the result of cellular oxidizing enzymes. BUT, if you press the tobacco, rupturing the cell membranes, as is done in press blocks, press cakes and Perique manufacture, then the cell contents of different tobacco varieties in the blend do meld, and if kept moist enough, will encourage microbial activity that significantly affects taste and aroma.
Rule of Thumb for Stoving Tobacco
There are no rules. Enjoy experimenting.
I did quite some experimentation on that matter - Bob just covered it.
I would underline that the key is in variation of moisture and temperature:
(Carefully) toast some in a dry pan --> not much darkening, but brings out chocolately notes and removes some undesirable flavours. (If you wonder how hot and for how long: Trust your nose).
In the oven in closed jars (leaves in case): Try 80 degrees Celsius, 120 degrees Celsius and 150 degrees Celsius for 3 hours and for 5 hours. The results about cover the range from Gawith's "Best Brown" over "Full Virginia Flake" to Rattray's "Dark Fragrant".
Open steaming (over a saucepan, for example): Has a mildening effect - too much, and things will end up as flavourless paper (esp. if the leaf is dripping).
In a pressure cooker in closed jars (leaves in lower-mid case, too wet and they become mushy) for 5+ hours: One should think there's not much difference to the oven method. My batches of Bolivian Criollo and Little Dutch became full-bodied and so strong, that I can only use them as a blending condiment.
Thank you Bob and Planter for summarizing all this in a few words !
I will keep those informations preciously.
I have tried cooking in the oven at 120°C in closed jars, but the results were not good at all... I think I used tobacco (Cherry Red 401) that was not old enough, it was only color cured and dried for a two month...
The result was not very good favorwise, and was nearly impossible to burn, it didn't want to catch fire...
I will try again when my tobacco will be a little more aged (now that I keep it in low/medium case, not dry).
Yes, it tends to burn (much) slower (which I think is one of the reasons Virginia is often treated that way). I had some high-sugar Prilep which turned almost black and doesn't burn well by itself anymore, but has a fantastic raisin aroma. Try mixing with some better burning stuff, and it will probably be delicious.
I did some red virginia in a jar in the oven yesterday - smells amazing, like stewed fruit and cherries, cant wait to have a bowl tomorrow. Hoping it will also alleviate the tongue bite I've been getting. The flavor of the straight red is great but I just cant tame its bite.
I'm no expert, but from my experience I couldn't tame bite on flue cured unless I mixed in a high ph tobacco like a burley or dark air or perique/fire cured(which is processed burley and dark air strains respectively)
I've tried mixing with Latakia and dark without much luck. I have some burley under pressure at the moment but that will be a few months away assuming it works out ok. I do have some fire cured which I've not experimented with yet. I'm not normally excessively prone to tonguebite but with the WLT blends I've been struggling.