Problems Caused by Acidity
Acidity is a problem for collectibles as it migrates into various materials and attacks the bonds, weakening them and causing a permanent irreversible change in many materials. Acidity is worst for fabrics and papers which are porous and which allow acid to migrate easily throughout the material.
High acid levels are often worsened by high humidity (which allows the acid to migrate more easily) and are very noticeable under strong light conditions, which may cause acidic papers and fabrics to yellow and turn brittle or disintegrate. For most collectibles, high acidity is visible in paint finishes, papers, and fabrics, and in corrosion of metals.
Whenever you create a miniature you should consider where it might eventually end up, and how it might affect other collectibles in its environment. Many miniatures are eventually housed in sealed roomboxes or cases to keep out dirt and dust. This can have the effect of containing acidity, which can damage many of the materials used to create the display.
Acidity migrates through items. This means acids in non acid free glues or papers or unsealed wood and mdf, can migrate into paper or fabric beyond where it touches the glue. If conditions are humid, acidity from a range of materials can become part of the local atmosphere as glues and materials offgas and begin to attack other materials including metals, stone, ceramics, paint, textiles, and leather. This can be especially problematic for miniatures in sealed surroundings, like plexiglass or glass fronted cases, where the environment can become more acidic as unsealed or unbuffered wood or miniatures offgas. The effects of light can also aid in creating a more strongly polluted environment which can also create more rapid breakdown.
Lignin is the main fiber in wood pulp based papers. Ground lignin can break down and create acidic conditions that turn paper yellow and brittle. This is the effect you see when acidic papers begin to age. To reduce the acidity in a display, reduce the unsealed sources of acid which offgas from soft vinyl based plastics, glues, lignin based papers and woods, especially those made with heavy quantities of glues, like plywood and mdf.
To ensure your miniature display lasts as long as possible, and inexpensive materials don’t damage important pieces of your display, avoid setting up acidic conditions in your miniatures. Do not use kraft paper or recycled papers unless you know they are acid free and lignin free. Avoid reusing materials such as cardboard egg cartons, which not only can be acidic, but may also contain residues from eggs, which are attractive to insects, and which may also contain salmonella.
Choose papers for wallpapers and printables which are acid free, lignin free and non buffered. These papers are not as likely to turn brittle with age, provided they are glued with acid free glues and have only acid free dyes, inks or paints used on them. Any acidic compounds, including sweat from bare hands, will soak into papers and textiles, and can migrate from an adjacent surface to the surface you place on or in it. Handle miniatures, especially paper miniatures and textiles, with clean hands, or wear cotton gloves.
Buffered papers are treated with alkali to neutralize acids as they migrate to the paper through handling or contact with non pH neutral glues and papers. Buffered papers may affect protein based textiles like wool and silk, or some photographs, and should therefore be avoided if possible, in favor of non buffered, acid free and lignin free papers.
Spray on archival mists and sprays can be used to buffer items where you are unsure if a special miniature is made of acid free papers, or where you are using a period paper. They help to neutralize the acidity in the paper they are applied to, but will not completely neutralize it or protect other items which touch it, as acid eventually migrates through the buffered layer. The buffering sprays can also create problems for photos or protein based fabrics like silk or wool.
Testing Materials for Acidity
Inexpensive pH test pens from art, photography and scrapbook suppliers can be used to text papers for acidity. The pen is used to make a mark on the paper, and the mark will change color, in a range from blue through green to yellow to indicate the paper's acidity. Remember that paper can absorb acid from your hands, so test areas of the paper which have not been handled, and also remember that paper if left in the atmosphere, can absorb acidity from its environment or from acidic materials it comes in contact with. Papers which start off acid free, may absorb sufficient amounts of acidity from the atmosphere to test as being acidic. Papers labelled as acid free may also contain lignin which can change the acidity of the paper as it breaks down. Lignin testing pens which turn orange on contact with lignin based papers are also available from museum and archival supply stores.