The science
The bar's action is soap chemistry with a targeted-contact modification. Sodium oleate, the sole surfactant, is the sodium salt of oleic acid, an unsaturated soap that lathers moderately and cleans by solubilising sebum in micelles at an alkaline pH, typically 9 to 10; like all true soaps it also removes intercellular lipids, swells corneocytes and raises surface pH transiently, effects documented in comparative studies of soap and synthetic-detergent cleansers as the source of dryness and irritation on normal and dry skin and experienced by oily skin as a thorough clean. Sorbitol and propylene glycol keep the bar translucent-soft and are why it absorbs water and softens in the jar. The enclosed textured sponge supplies the exfoliation; the bar itself contains no abrasive, and the activated charcoal is a fine adsorbent powder that binds oil and pigment in the lather, has no demonstrated effect on follicular contents during a wash, and accounts for the colour.
The acne actives are recognised agents used below their recognised doses. Salicylic acid is listed in the FDA acne monograph at 0.5 to 2 percent and sulfur at 3 to 10 percent; the absence of a drug facts panel places both below those levels. Salicylic acid penetrates the sebum-filled follicle in its un-ionised form, which predominates below its pKa of about 3, and in an alkaline oleate soap it is almost entirely ionised and confined to the surface, where during a one-to-two-minute mask it adds a mild keratolytic effect; sulfur contributes weak keratolytic and antimicrobial activity at the surface on the same timescale. Lactic acid at the end of the list adds a trace of alpha-hydroxy activity, likewise limited by pH and contact. Eucalyptus leaf oil provides eucalyptol and terpenes with weak antimicrobial activity and a recognised sensitisation potential; turmeric oil contributes turmerone and colour.
The even-tone ingredients have leave-on evidence only. Niacinamide reduces melanosome transfer to keratinocytes and has controlled-trial support at 2 to 5 percent applied daily for weeks; its fifth-position inclusion here is meaningful for a bar and its exposure during a short mask is a small fraction of that regimen. Kojic acid chelates the copper in tyrosinase and works at 1 to 4 percent over months of continuous contact; at tenth on this list for minutes it has no plausible effect on pigment and retains its sensitisation potential. The realistic mechanism by which the bar improves body marks is prevention, fewer inflammatory lesions producing fewer post-inflammatory macules, rather than any direct depigmenting action, and Urban Skin Rx publishes no testing that would show otherwise.