After a 36-hour HVAC failure spiked the storage room to 78% RH at 22°C, I’ve isolated a 19th-c. oil on canvas, returned it to 50% RH, and surface-vacuumed through a screen; before I trial 70% ethanol (IMS):deionized water on test areas, I’d value any experience on pigment sensitivity and reactivation risk. Do you favor strictly mechanical removal plus time at controlled RH over alcohol-water applications on low-gloss, oil-rich surfaces, and if swabbing is warranted, which contact time and wick materials have given you the least tide-line formation?
On a similar 1890s oil we kept 50% RH for a week and went purely mechanical — micro-HEPA plus a very light vulcanized rubber ‘smoke’ sponge through a screen under magnification — which cleared the bloom without the faint dye bleed I’ve seen on Prussian blue/alizarin when testing 70% IMS. If you do try solvent, start barely damp at about 90% IMS on discreet losses and avoid soft matte retouch/oxidized varnish; @m.klein, the AIC notes are handy: Category:Mold/Fungi - MediaWiki. Mold’s like a bad roommate — stop feeding it and it moves.
I’ve had 70% IMS:water wake up a megilp-rich glaze and nudge Prussian blue, so I now ring the test area with a thin blotter collar to wick moisture and work at 55–60% IMS on a swab under gentle local suction through the screen — if the swab drags, I stop and give it another 48 h at 50% RH. @m.klein is right on controlled RH, and the AIC notes help: Category:Mold/Fungi - MediaWiki; small caveat — skip smoke sponge on friable matte passages.
After you’ve stabilized back to “50% RH,” I’ve had better luck delivering IMS:water through Gore‑Tex (solution on the scrim side, membrane to paint) so you get vapor only; the bloom powders and lifts cleanly under micro‑HEPA without waking fatty glazes. On a 19th‑c. canvas after a “36‑hour” spike, this avoided tidelines entirely, but watch for slight blanching on very lean varnish and stop if saturation jumps.
Echoing @lhawkins89 on keeping moisture tight: after a 36‑hour about 80% RH blip on a varnished 1880s oil, a short pre‑dry at 45–50% with gentle panel warmth (about 30–32°C) made the bloom friable and reduced smearing. Under mag I used light Evolon dabs through the screen and, only if residue clung, a barely‑damp swab with a 60:40 ethanol:isopropanol mix — fewer surprises with blues than straight 70% in water. Caveat: if it’s wax‑resin lined, skip the warmth to avoid mobilizing the adhesive.
On a 19th‑c canvas that hit about 78% RH at about 22°C, I kept solvent from waking glazes by working on a low‑draw suction platen — , it’s the only way I’ve dodged surprises… IMS:water around 60–65% on Evolon with just enough draw to see a faint wick kept Prussian blue and megilp calm and prevented “tide lines”; if the varnish starts to grumble, I switch to lightly gelled ethanol (Klucel‑G). If you don’t have suction, @lhawkins89’s vapor‑only idea is the next safest, but mask the area with a Mylar window so the moisture can’t creep.
Have you checked for bitumen/asphaltum in the dark glazes — , that’s where I’ve had the worst surprises. In those spots, after your screen‑vac I’d either stay purely mechanical for a bit at 50% RH or go very ethanol‑rich (≈90:10 ethanol:DI) and roll the swab rather than drag to keep “reactivation risk” down. Small caveat: lake reds can tint a swab even at high ethanol — work in raking light and stop at the first gloss shift, @nathaniel_77.
I’ve had good results gelling the ethanol (about 2% Klucel G) and applying through thin Gore‑Tex to limit wetting; it trims back hyphae without waking transparent layers. If you suspect a natural resin varnish, I’d stay dry and let it sit near 50% RH, since even 70% can leave it matte — AIC’s mold page has a solid rundown: Category:Mold/Fungi - MediaWiki. Treat solvent like hot sauce — less is more.