Yellowing is the bane of any paper archivist. Paper constructed before the Industrial Revolution was typically handmade from the strong fibres of impurity-free cotton rags and sized for printing with chemically inert animal gelatin; paper made today (as of this publication) is usually formed from the short, weak fibres of bleached and shredded wood pulp and is full of acids and other impurities. Storage next to non-archival mats, cardboard and in direct sunlight add to the yellowing of all modern newspapers, letters and prints due to oxidation, but there is a solution. The process of de-acidifying paper in an appropriate solvent removes most of the yellow discolouration and is possible to do at home.
Add 2 tbsp. Milk of Magnesia to 4.25 cups of soda water. Let the mixture sit for eight hours. Pour .75 cup of the blend into the flat pan. Submerge the paper in the solution; soak for one hour. Change the solution each time the mixture turns yellow.
Put on rubber gloves. Lift the paper carefully out of the pan by holding two corners between the thumbs and index fingers of either hand. Drape the folded crease over one rang of the plastic drying rack.
Put on chemical goggles, two pairs of rubber gloves and full clothing. Add 4.25 tsps. pure sodium chloride crystals and a scant teaspoon of formaldehyde to a 4.25 cups of distilled water. Pour the solution into the flat pan in a well-ventilated area. Soak the yellowed paper in the pan until the water turns yellow.
Add clean distilled water to a second flat pan. Grasp two corners of the paper between the thumb and index finger of both hands and lift gently out of the pan. Switch the grip on the paper: hold the centre of the two short sides of the document between the thumbs and index fingers. Dip the right side of the paper into the left side of the pan and slide to the right to submerge the entire paper. Lift out of the water leading with the right side. Rinse twice more. Lay the document on blotting paper, change the water in the rinsing pan and rinse three more times.
Lay a sheet of clean cotton fabric over an ironing board. Place a sheet of blotting paper over the cotton and place the thick, wet paper face down on the blotting paper. Lay a second piece of blotting paper over the back of the print and the other sheet of cotton fabric on top. Heat a dry iron to medium-hot. Iron over the covered print in a continuous, smooth motion for 5 to 10 minutes. Move the iron to the side, uncover the print and place it face up on a third piece of dry blotting paper. Allow to air-dry for 1 hour. Pour the cleaning mixture into a glass jar and dispose of at a hazardous waste facility.
Prevent paper from yellowing or becoming brittle by displaying under UV-protecting glass to shield from light damage, using archival mats and containers to store and spraying with archival de-acidifying solutions made with methoxyl magnesium methyl carbonate.
Formaldehyde exposure is dangerous --- keep away from children, wear eye and body protection when handling and work in a well-ventilated area.