MicroChamber/Silversafe and Lig-free Type II boards for the Preservation of Archeological Specimen, Photographic Materials and Textiles
We have developed and currently manufacture two archival boards which we are using to make containers for the storage and preservation of photographic images and proteinaceous artifacts such as ancient skills, parchments and leathers, textiles (silks and wools), anthropological artifacts including insect collections, horn, bone, hides, teeth, feathers and albumin and gelatin emulsions commonly used in photographic prints and negatives. Before we describe our new boards, we would like to briefly explain the structure and composition of proteins and the process by which they are assembled in a living cell. Outlining these fundamentals of protein structure and function will give people a better understanding of our reasons for developing these boards.
A protein is a long chain (polypeptide) of x-amino acids (2-amino carboxylic acids) which fold into various three dimensional conformations, thereby controlling access to the reactive chemical groups in the particular three dimensional patterns. There are 20 different amino acids which can be placed end to end in any order to comprise the links of this polypeptide chain. Unlike cellulose, which is comprised of a long chain of identical glucose rings, the protein can exhibit considerable diversity because of the vast numbers of sequences of amino acids which are possible and the fact that each of these amino acids has a different reactive group called a side chain, which is the primary determinant of the property of a given protein. Also, protein molecules are often comprised of not one, but several different polypeptide chains.
The construction of a protein begins when an enzyme (which is also a protein) called an RNA polymerase uncoils the DNA double helix in the nucleus of a cell, exposing six bases or nucleotides at a time. Each three adjacent nucleotides (called a codon) designates, or codes for, a specific amino acid. Heterogeneous nuclear RNA (hnRNA) is formed using the DNA template to acquire a complimentary sequence of nucleotides to those found in the gene (DNA) being expressed.
Messenger RNA (mRNA) formed from the hnRNA leaves the nucleus of the cell with the precise number and sequence of nucleotides required to code for the specific protein called for by the DNA (gene) in the nucleus of the cell. The mRNA travels through the cytoplasm of the cell to a site comprised of four molecules of RNA and many different proteins called a ribosome.
In the ribosome transfer RNA, consisting of three nucleotides (a codon) and the amino acid specified by the codon, attach to three complimentary nucleotides on the mRNA. As the ribosome moves sleeve like down the length of mRNA, the transfer RNA continues to combine to the next three complimentary nucleotides on the mRNA bringing the next amino acid to the growing polypeptide chain. As the amino acid carried by the tRNA is attached to the polypeptide chain, the tRNA is released to the cytoplasm where it combines with another amino acid. This process continues with the polypeptide chain of amino acids growing longer and longer until the ribosome comes to a terminator sequence on the mRNA. The completed polypeptide is detached into the cytoplasm where it folds into its specific three dimensional conformation and becomes a protein. It is when it is in this final three dimensional conformation that it exhibits those properties we associate with this specific protein.
The side chains of the amino acids are ionic and, therefore, form electrostatic bonds between each other which hold the protein into its particular conformation. Changes in pH affect the ionic forms in which the side chains of the amino acids exist and, therefore, changes in pH also affect the formation of bonds by proteins. In order for an electrostatic bond to exist between side chains, both positive and negative changes must be present. Raising the concentration of H+ (increasing the acidity) decreases the number of charged caboxylate ions and the carboxylate group on the side chains lose their charge. If the concentration of H+ is lowered (made alkaline) the H+ will leave the ammonium group which will lose its charge. It follows then that these bonds are generally most stable near neutrality. Also, for these amino acids with both carboxylate and ammonium groups, there is a pH value at which the number of negatively charged carboxylate groups will be exactly the same as the number of positively charged ammonium groups. Various points along the side chain will be more negative or more positive than others which allows the bonding to continue, but the total charge will be zero. This is the isoelectric point of the protein and the pH at which this isoelectric point occurs is where the protein is least reactive and, therefore, most stable.
If the pH, or isoelectric point, of the protein is altered, the side chains of the amino acids comprising the protein will lose their ionic forms and the electrostatic bonds between them will be broken. The protein can then unravel from the distinct form which gave it the properties we associated with this specific protein and become a polypeptide again. In addition, the peptide bonds between the amino acids are now exposed to the possibility of clevage by hydrolysis. By comparison, if we break the covalent bonds in a cellulose chain, we do not alter the properties we associated with the paper. It looks the same and feels the same. It just gets weaker.
This sensitivity and the potential magnitude of damage, coupled with the persistence of several conservators and conservation scientists, prompted us to look for an alternative to highly alkaline buffered papers for the long term storage of proteinaceous artifacts. Our objective was to invent an archival material from which we could make containers that would in no way harm or interfere with the artifact stored within it. A very pure, neutral, non-buffered paper which was itself at its isoelectric point would provide precisely the neutral, non-reactive environment we wanted. We added a thick layer of alkaline pH, alkaline buffered paper to the neutral pH, non-buffered paper which forms the interior of our containers. These boards, which we call Lig-free Type II, provide a securely neutral, non-reactive, sulfur free interior while the outer plies of the board contain alkaline buffering. This was the first, and until recently, the only boxboard available which addressed the needs of collections felt to be sensitive to an alkaline environment. We added a thick layer of alkaline pH, alkaline buffered paper to the neutral pH, non-buffered paper which forms the interior of the containers.
Lig-free Type II is currently available in a corrugated board. The outer liner and inner corrugated medium are alkaline pH, alkaline buffered paper while the inner liner is a neutral, non-buffered paper. This board is strong, exceeding 250 lbs. pressure per square inch bursting strength, and rigid. It is especially good for making large textile boxes which can then be shipped and stored flat, for backing boards, and for use as a support in conservation work. In Lig-free boards, all the buffered papers are light tan in color while the non-buffered papers are white, so it is easy to see which side is buffered and which is not buffered. All the papers used in these boards, both buffered and non-buffered are pure long chain cellulose and they are free of lignin, sulfur and other deleterious substances. Complete specifications are detailed under the headings Lig-free Type II (alkaline pH, buffered paper) and Photographic/Textile Conservation Paper (neutral pH, non-buffered paper).
Now we also offer MicroChamber/Silversafe products, the first boxboards and papers able to address both the needs of alkaline sensitive collections, and the shortcomings of alkaline-buffered-only archival storage products in dealing with pollutants produced both indoors and outdoors, and by-products of deterioration. Standard MicroChamber boxboards combine alkaline buffers, activated carbon and zeolite molecular sieves, offering your collection the greatest protection available from acids, by-products of deterioration and pollutants. Now MicroChamber/Silversafe boxboards provide a combination of buffered Lig-free board for strength, thickness, and support, coupled with tan MicroChamber paper faced with a surface of neutral pH, unbuffered, soft white cotton Silversafe paper. These products were developed so the advantages of MicroChamber technology could be combined with neutral pH, unbuffered cotton Silversafe paper, for use with protein-based textiles such as silks and wools, and with certain photographic materials, and other artifacts which collection managers feel may benefit from the neutrality of unbuffered cotton coupled with the protective security offered by MicroChamber materials. MicroChamber/Silversafe products are currently available in folders, folder paper, negative enclosure paper and envelopes, as well as solid-fiber and corrugated boxboards, backing boards and support boards.