Why Is My Handmade Paper Growing Spots or Smelling Musty?

Spots have appeared on a sheet, or a stack has a musty smell. A note on the word: the papermaking mould is the screen you form sheets on, and every other "mould" in this guide means fungal growth.

Before you change anything, sort out what the spots are. Growth develops over time on damp paper. Specks from the pulp are there from the start.

• Growth often appears after the sheet has been damp for days, can look fuzzy, and may spread.
• Specks from the pulp can be seen on the wet sheet and do not change. If you recycle printed scrap, the guide on dingy recycled paper covers specks from the source paper.

This site gives no health advice. If you have concerns about exposure to mould, ask a medical professional. For guidance on handling mouldy paper, read the leaflet from the Northeast Document Conservation Center listed under Sources.

Change one thing per batch, and keep notes.

1. Look at a wet sheet. Are the specks already there? If so, the problem is in the vat, not the drying.
2. Count the days. How long did the sheets stay damp? A stack that has sat damp for several days is the likely cause.
3. Check the layers. Were the blotters or felts changed daily? Are any of them still damp or musty?
4. Smell the felts. Inspect them for dark spots. Bannister notes that mildew shows up on felts as dark spots.
5. Check the room. Is the air still? Is the room humid? Is the stack on the floor or against a cold wall?
6. Check the storage. If the sheets were clean when dry, look at where you keep them.

Speed up the drying

Conservators treat moisture as the main driver of growth. The Northeast Document Conservation Center notes that mould normally grows on wet materials in about 48 hours, sometimes sooner. A damp stack can get there quickly.

Change blotters or felts every day. Helen Hiebert warns that sheets may mould if the absorbent layers are not replaced. Make: gives the same advice for sheets that take more than a day to dry.

Move the air. NEDCC notes that stagnant air lets airborne spores settle, and its salvage advice brings in fans and a dehumidifier to get the relative humidity to 55% or lower.

Do not reach for heat

NEDCC warns against turning up the heat when humidity is high, because extra heat can make mould grow faster. Move air and lower humidity instead.

Clean or replace the felts

Bannister washes felts in mild soap and ammonia and hangs them in the sun if possible, to discourage mildew. Replace any that stay musty.

Keep bad sheets out of the vat

NEDCC's first step with mouldy material is to isolate it from the rest of the collection so spores do not spread. Treat a sheet with active growth the same way, and keep it away from the vat and your clean sheets.

Keep the storage dry

NEDCC's general advice for paper is moderate, steady conditions: below 70°F and below 60% relative humidity, with good air circulation, away from damp places. Keep paper off the floor and away from cold outside walls. Basements are a poor choice because they are often damp and have little air movement.

• Keep drying short. Change damp layers daily and write the changes in your batch log.
• Measure the humidity in the drying room. NEDCC recommends an instrument that reads relative humidity and temperature accurately.
• Clean and dry felts before you store them. Bannister stores dry felts in plastic bags.

A sheet with visible growth is rarely worth saving. Staining and damage to the fibres usually remain after the growth is gone. NEDCC notes that active mould produces enzymes that alter, weaken, and stain paper.

If you need to deal with an important sheet, or a whole stack of mouldy paper, follow NEDCC's guidance or ask a conservator. Do not spray bleach or household cleaners on paper. NEDCC warns that quick cures of that kind can cause more damage and are often ineffective.