Xeroform dressings turn black because of a chemical reaction in their active ingredient, bismuth tribromophenate. When this compound comes into contact with air, moisture, and wound drainage, it oxidizes and darkens, shifting from its original yellow-gold color to brown and eventually black. This is a normal chemical process, not a sign of infection or a problem with your wound.
What Causes the Color Change
Xeroform gauze is impregnated with a petrolatum base containing 3% bismuth tribromophenate, which gives the dressing its antimicrobial properties. Bismuth compounds are reactive to their environment. When the dressing sits on a wound, it absorbs moisture from wound fluid, blood, and sweat. That moisture, combined with oxygen in the air and the warmth of your body, triggers oxidation of the bismuth. The result is a progressive darkening that can range from deep brown to jet black.
The speed of this color change depends on how much drainage your wound produces and how long the dressing stays in place. A dressing on a heavily draining wound can turn noticeably dark within hours. On a drier wound, it may take a day or more. Either way, the darkening itself is purely cosmetic and does not affect how the dressing works.
Black Dressing vs. Signs of Infection
It makes sense to worry when you see something black on a wound, but the key distinction is straightforward. Bismuth oxidation changes the color of the dressing material itself. The gauze turns dark uniformly, and the wound underneath looks the same as it did before. Infection, by contrast, produces changes in the wound and surrounding skin: increasing redness that spreads outward, warmth and swelling around the wound edges, thickening or foul-smelling drainage, and worsening pain rather than gradual improvement.
If you peel back the Xeroform and the wound bed looks healthy (pink or red tissue, no unusual odor, no pus), the black color on the gauze is simply oxidized bismuth doing what it does.
Dark Staining on Your Skin
Bismuth oxidation can also leave temporary gray or dark stains on the skin around your wound. This happens when the oxidized compound transfers from the gauze onto your skin surface. The staining is superficial and fades on its own over days to weeks once you stop using Xeroform. Gentle cleansing during dressing changes can minimize buildup, but scrubbing at healing skin is not worth it. The stains are harmless.
When to Change the Dressing
The darkening of Xeroform is not itself a signal that the dressing needs to come off, but it does tend to coincide with the dressing drying out and losing its non-stick petrolatum layer. Once that happens, the gauze can adhere to the wound bed and cause pain or tissue damage when removed. For skin grafts, surgical protocols from McGovern Medical School call for the first Xeroform layer to stay in place for two to three days after surgery, then switch to daily dressing changes with fresh Xeroform and antibiotic ointment.
For general wounds, most care instructions call for changing Xeroform every one to two days, or sooner if the dressing becomes saturated with drainage. If the gauze feels dry and stiff rather than soft and waxy, it has lost its moisture barrier and should be replaced regardless of color. Soaking a stuck dressing with sterile saline or clean water for a few minutes before removal helps prevent pulling on new tissue.
Other Bismuth Products That Darken
This same oxidation reaction is not unique to Xeroform. Bismuth subsalicylate, the active ingredient in Pepto-Bismol, can turn your tongue and stool black through the same basic chemistry. When bismuth meets sulfur compounds (present in saliva, gut bacteria, and wound environments), it forms bismuth sulfide, a deep black compound. In Xeroform specifically, the combination of sulfur from wound proteins and oxygen from air exposure accelerates the darkening. If you have ever taken Pepto-Bismol and been alarmed by a black tongue, the mechanism on your wound dressing is essentially the same.