How to induce artificial gangrene?

Have Torturer Bob start looking at our friend the Brown Recluse Spider (https://en.wikipedia.org/wiki/Brown_recluse_spider). Brown Recluse spider bites can occasionally cause a sort of progressive tissue necrosis which is similar to small scale Gangrene.

So Torturer Bob starts talking to to Sally in the mad science department and they come up with a way to synthesize the recluse venom.

Here is how it becomes a rather nasty torture: The bite of a brown Recluse often goes totally unnoticed. It then starts to itch, followed by pain. As the tissue dies off, nerves become exposed before they die, much like a 2nd and 3rd degree burn. Ouch. You have all sorts of options when the victim has some open sores. You have the added benefit of being able to keep control of the level of infection and so on, unlike with massive sepsis that can happen with Wet or Gas Gangrene.

I think this question hits the very limits of good taste. I couldn't find a proper category to flag it into, though, so here is a brief answer about what gangrene is and how it comes into being. And the only way I could think of to make it controlled all the way to death.

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All quotes are from Wikipedia.

Gangrene (or gangrenous necrosis) is a type of necrosis caused by a critically insufficient blood supply.

Dry gangrene

Dry gangrene is a form of coagulative necrosis that develops in ischemic tissue, where the blood supply is inadequate to keep tissue viable. Dry gangrene is often due to peripheral artery disease, but can be due to acute limb ischemia.

Acute limb ischaemia (ALI) occurs when there is a sudden lack of blood flow to a limb.

Make turniquet on limb to cut off blood flow. Control progression by applying tourniquet a bit higher once the tissue below is dead. A human has four limbs that can be treated that way. If that still isn't enough sadism, men have genitals and women have breasts.

Wet Gangrene, gaseous gangrene

Wikipedia:

Wet, or infected, gangrene is characterized by thriving bacteria and has a poor prognosis

Gas gangrene is a bacterial infection that produces gas within tissues.

Not recommended if you want to keep your prisoner alive for a long time

due to sepsis resulting from the free communication between infected fluid and circulatory fluid.

Sepsis is a life-threatening condition that arises when the body's response to infection injures its own tissues and organs.

Trying to keep a patient with sepsis alive is quite hard, and it always necessitates treating the origin of the sepsis -- the wet / gaseous gangrene you just tried to induce. Too much effort and too much chance involved to make a good torture method out of it.