The National Archives & Records Administration (NARA) has been making heat-set mending tissue in house for many years. Recent digitization initiatives have increased the need for efficient stabilization mending. NARA prefers heat-set tissue for this type of mending for many reasons. The tissue is flexible and easily reversible. It requires no moisture for application and is easy to use. The transparency of the tissue, which they can control by making the tissue in house, does not interfere with the digitization of text. To make the heat-set tissue, NARA starts with an appropriate weight of Japanese tissue, which is toned (if necessary) before application of the adhesive. The tissue is wetted and smoothed out against silicone Mylar to remove bubbles. A batch of the acrylic emulsion polymers Rhoplex AC 234 + AC 73 were mixed and applied through a screen onto the wet tissue. The tissue was then allowed to dry on the Mylar until ready for use. Unfortunately, the Rhoplex adhesives they had been using for many years have been discontinued, and they had to search out a new blend of adhesives to continue making the tissue. NARA tried two different blends of adhesives: Avanse MV-100 + Plextol B500 and Avanse MV-100 + Rhoplex M200. NARA settled on a 4 : 1 : 1 ratio of water : Avanse MV-100 : Plextol B500 for their new mix. PROS:
- FTIR analysis showed that the adhesive, when applied through a screen, does not sink through the Japanese tissue.
- Blocking tests also showed the tissue safe to use on multiple layers of documents.
- The mixture passed the PAT test for use on photographs.
- Avanse MV-100 has optical brighteners in it, which is something of a concern. Advanced aging test showed that the optical brighteners did not migrate into the documents which had been mended, however, so it was deemed acceptable for use.
- The tissue also has a high sheen from the silicone Mylar that can be objectionable to some clients. It isn’t bad enough to cause problems for digitization, however, and it can be removed with a swab of alcohol if necessary.