Photochemicals can be bought in liquid or powder form, needing diluting and dissolving.
Other chemicals are also used in black and white processing include: formaldehyde as a pre-hardener, a variety of oxidizing agents as hypo eliminators, sodium sulfide, silver nitrate, solvents for film and print cleaning and concentrated acids to clean trays.
Electrical outlets and equipment present hazards due to the risk of splashing water.
- Concentrated sulfuric acid, mixed with potassium permanganate or potassium dichromate, produces highly corrosive permanganic and chromic acids.
- Hypochlorite bleaches can release highly toxic chlorine gas when acid or heat is added.
- Potassium persulfate and other hypo eliminators may cause fires when in contact with many solvents and other combustibles. Most are also skin and eye irritants.
- Cleaning acids should be handled with great care. Wear gloves, goggles and acid-proof, protective apron. Always add acid to the water when diluting.
- Do not add acid or heat to hypochlorite bleaches.
- Keep potassium persulfate and oxidizing agents separate from acids and flammables.
- Install GFCIs whenever electrical outlets are within six feet of water.
Stop baths are a weak solutions of acetic acid. Acetic acid is commonly available as pure glacial acetic acid or 28% acetic acid. Some stops contain potassium chrome alum.
Fixing baths contain sodium thiosulfate (“hypo”) as the fixing agent and sodium sulfite and sodium bisulfite as a preservative. Fixing baths may also contain alum (potassium aluminum sulfate) and boric acid.
- Acetic acid is highly toxic by inhalation, skin contact, and ingestion. It can cause dermatitis and ulcers and can strongly irritate the mucous membranes.
- The final stop bath is only slightly hazardous by skin contact. Continual inhalation of acetic acid vapors may cause chronic bronchitis.
- Potassium chrome alum is moderately toxic, causing dermatitis and allergies.
- In powder form sodium thiosulfate and bisulfate are not significantly toxic by skin contact. By ingestion it has a purging effect on the bowels. Upon heating or long standing it can decompose into highly toxic sulfur dioxide, causing chronic lung problems. Many asthmatics are particularly sensitive to sulfur dioxide.
- Alum (potassium aluminum sulfate) is only slightly toxic; skin allergies or irritation.
- Boric acid is moderately toxic by ingestion or inhalation and slightly toxic by skin.
- All darkrooms require good ventilation to control acetic acid and sulfure vapors.
- Wear gloves and goggles if concerned about exposure, especially expectant mothers.
- Cover all baths when not in use to prevent evaporation of toxic vapors and gases.
Toning a print involves replacement of silver by another metal, for example gold, selenium, platinum, or iron. In some cases, the toning involves replacement of silver metal by brown silver sulfide.
- Sulfides release highly toxic hydrogen sulfide gas during toning or when treated with acid.
- Selenium is a skin and eye irritant and can cause kidney damage. Treatment of selenium salts with acid may release highly toxic hydrogen selenide and sulfur dioxide gas.
- Gold and platinum salts can produce allergic skin reactions and asthma.
- Carry out normal precautions for handling toxic chemicals as described in previous sections. In particular, wear gloves and goggles. See mixing section.
- Toning solutions must be used with local exhaust ventilation.
- With two bath sulfide toners, make sure you rinse the print well after bleaching in acid solution before dipping it in the sulfide developer. Make sure that sulfide or selenium toners are not contaminated with acids.
- Avoid thiourea whenever possible because of its probable cancer status.