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C.M.C (Carboxy Methyl Cellulose)Carboxymethyl cellulose (CMC) or cellulose gum[1] is a cellulose derivative with carboxymethyl groups (-CH2-COOH) bound to some of the hydroxyl groups of the glucopyranose monomers that make up the cellulose backbone. It is often used as its sodium salt, sodium carboxymethyl cellulose.
Methylcellulose & Sodium Carboxymethylcellulose: Uses in Paper Conservation by Cathleen Baker
This paper concerns the many uses of these two polymers in conservation. Although this information is paper conservation oriented, it may very well be that it can be useful to conservators in other fields.
Methylcellulose (MC) and sodium carboxymethylcellulose (sod. CMC) have many other uses besides those in conservation. A brief rummage through your medicine cabinet may come up with such products as toothpaste, laxatives, or diet pills each of which may contain either MC or sod. CMC. Other products include ice cream, water-based paints, detergents and a variety of paper products to name but a few. Characteristics which make them useful are: high viscosity in low concentrations, defoaming abilities, surfactant, and bulking abilities. They are not toxic and do not promote allergic reactions in humans. At the end of this paper, I have given more technical information on the composition of the various MCs and sod. CMCs.
These cellulose polymers can be purchased in grades ranging from coarse to fine particles and in varying viscosities. In solution, Hercules CMC 7H and Culminal (MC from Talas) are quite clear while Cellofas B3500 from Conservation Materials is hazy.
The easiest way to make up any of the MCs or sod. CMCs is to measure out the desired quantity of the powder, fill a blender with the right amount of deionized or distilled water, turn on the blender to the lowest speed, and pour the powder in a steady stream into the vortex. As soon as all of the powder is in the water, turn off the blender. Over-blending can result in a loss of viscosity. No preservative is necessary as long as purified water is used and the storage container is airtight. After blending, it is best to leave the solution at least one hour before using.
Most conservators regard the MCs and sod. CMCs primarily as adhesives. We will see in fact that they have many other uses, but let’s start with their adhesive applications. First may I say that neither MC or sod. CMC alone is really strong enough to be used as an adhesive when a great deal of stress on the bond is encountered such as in tear repairs or hinges.
MC should also not be used for an overall backing as it is not a very polar adhesive and as such will not affect a very good bond between papers, especially smooth-surfaced papers. It is occasionally mixed with wheat starch paste in order to provide ‘slip’ and indeed this mixture comprises most wallpaper pastes. Sod. CMC on the other hand is a very polar adhesive and as such makes a very good bond between sheets of paper and is useful for overall backings where stress on the bond is not a real problem. It could also be mixed with wheat starch paste to provide ‘slip’. The advantage sod. CMC has over wheat starch paste is that a dry backing can be done when the original work of art consists of water sensitive media or paper which is dimensionally unstable. This is because a 2.5% solution of say CMC 7H is very viscous but is not very ‘wet’ so that it can be brushed on the reverse of the original and the backing paper without either expanding too much or without much water penetrating to the surface of the original. Once the backing is complete, air drying can take place with the original face down so that water evaporates from the back reducing still further the risk of the front getting wet. When dry the backed original can be humidified and pressed or put out on a drying screen/board to flatten. The adhesive dries to a very thin even layer and is easily reversible with cold water. It is non-staining and does not become brittle upon ageing. Other applications in this section might include temporary facings, mends or backings. If you can use hot water for a wash bath, you might like to use MC as the temporary adhesive as it will not dissolve in hot water but is easily reversed in cold water. This would probably hold together a badly torn piece which could not be handled in any other way.
An extremely useful method for quickly filling small holes or losses in paper is accomplished by mixing Whatman Cellulose Powder CF11 with MC or sod. CMC. The cellulose powder as sold is very white, but gradations of browns can be made by cooking the dry powder in a Teflon pan over a hot plate. Wear a dust mask as the powder can be irritating if inhaled. Also be careful not to scorch the powder. The cooked powder can then be color matched, dry, to the original paper by adding lighter or darker shades of the powder as necessary. Then enough 3% MC or 2.5% sod. CMC is added to make a stiff paste. It can then be applied to the hole or loss with a microspatula, tip of a scalpel blade, etc. Leave it to air dry. Retouching is seldom necessary if you have matched the powder color well in the first place. If a smoother surface is needed, small amounts of calcium carbonate can be added to the powder before the adhesive is added. Any excess of the cellulose powder paste left over can be allowed to dry out, and later on, rejuvenated for further use by adding a few drops of water and working up into a paste again.
These cellulose polymers also act as deflocculating agents in that they cause particles such as fibers to stay in suspension and not clump together and separate out of solution. This advantage can be very useful in employing wet pulp fills in a treatment, especially when using a pipette to distribute the pulp from the slurry. The proportions are about 1:3, .5% MC or sod. CMC: pulp slurry.