Encapsulation in Polyester Film using Double-coated Tape
Encapsulation is a simple technique designed to protect documents from physical wear and tear as well as grime and pollution. After encapsulation, even a brittle document can usually be handled safely (Fig. 1).
Figure 1. top, Document is encapsulated between two layers of uncoated polyester film. Double-coated tape
bonds the two layers of polyester film around the document.
bottom, Detail showing two layers of polyester film bonded with double-coated tape.
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Figure 1: Eileen O’Hara
Figures 2 – 8:
Margaret R. Brown
The document is enclosed between two sheets of clear polyester film, the edges of which are sealed with double-coated pressure sensitive tape. The process is easily reversed by carefully cutting the film envelope along the edges in the space between the tape and the document.
Materials Tools Supplies
- Grid work surface (Fig. 2) [can be prepared by taping 1/4in graph paper to the underside of a sheet of glass or sheet of polymethylmethacrylate]
Figure 2. Grid work surface.
- Hard rubber brayer
- Lint-free cloth (cheesecloth)
- Low lignin buffered paper, similar in weight to the document
- pH neutral paper board (optional)
- Polyester film, pre-cut or in rolls 3-4mil (small documents), 5mil (large documents)
- Polyester tape, double-coated, acrylic-based adhesive
- Scalpel or knife or good scissors
- Window-cleaning squeegee
- If a backing sheet of alkaline reserve paper is appropriate, cut the sheet of paper to the dimensions of the document.
- Cut two sheets of film at least one inch larger than the document in each dimension.
- Place one sheet of film on a flat work surface. Wipe the surface of the film with a lint-free cloth to remove dust and improve the static charge, which will adhere the film to the work surface.
- Center the backing sheet, if used, on the film and place the document on top of it.
- Place one weight on the center of the document to keep it in position (Fig. 3).
Figure 3. Document held in position with weight.
- Leave the brown protective paper on the tape, and apply the tape to the film along the edges of the document, leaving a space of 1/8 – 1/4in in between the edge of the tape and the edge of the document (Fig. 4).
Figure 4. Double-coated tape is applied around the perimeter of the document on the polyester film.
The ends of the tape should be cut square and butted on three corners with no overlap (Fig. 5, left).
Figure 5. Tape joins. left, Butted with no overlap. right, Mitered.
Alternatively, the tape edges can be cut on the diagonal to make a more elegant joint (Fig. 5, right). Leave a gap of at least 1/16in at the fourth corner to allow air to escape.
- Wipe the second sheet of film with lint-free cloth.
- Remove the weight from the document and center the second sheet of film over assembly, cleaned side down.
- Replace the weights on the center of the top sheet of film.
- Lift one corner of the top sheet of film at a time and carefully peel the protective paper from the tape along one edge of the document. Lower the corner of the film and rub the film over the tape to adhere it (Fig. 6). Repeat for the other three edges.
Figure 6. The top layer of polyester film is lifted and the protective paper from the tape is removed.
- To remove air from between the sheets of film, slide a squeegee across the envelope towards the air gap left in one corner of the envelope (Fig. 7).
Figure 7. Squeegee is used to removed air from between the sheets of polyester film.
- Roll the brayer or squeegee over the taped portion to bond it firmly to the polyester, or run your finger over the tape to secure the bond (Fig. 8).
Figure 8. Encapsulation tools. left, Squeegee. right, brayer.
- Trim the envelope, leaving a 1/8 – 1/4in margin of polyester film outside the tape on all four sides. Rounding the four corners will help prevent scratching or cutting other materials during handling.
One should practice encapsulation techniques several times on unimportant objects before working on a valuable document. This technique should be used only for flat paper.
Encapsulated documents are held in place between the film layers by static electricity. The static helps also to hold torn paper together, reducing the need to repair small tears before encapsulation.
However, the static can lift loosely bound media from the paper. For this reason, the technique is inappropriate for documents with media that are not firmly bound to paper, such as pastel, charcoal, and some pencils.
If in doubt, test unobtrusively; if the media lifts off on a tiny swab rubbed gently in an inconspicuous spot, the document should not be encapsulated.
Research by the Library of Congress (1980) demonstrates that acidic papers may age much faster after encapsulation. It further shows that leaving an air space at the corners of the film package does not slow this aging.
Documents should be alkalized by a qualified person before encapsulation. If this is not feasible, encapsulation may still be desirable to protect very fragile or heavily handled material. The Library of Congress finds that in such a case, using a sheet of alkaline reserve paper the same size and shape as the document to back it in the enclosure can slow down the rate of deterioration. Documents that are not alkalized before encapsulation should be labeled for future custodians. A label typed on alkaline reserve paper and inserted in the envelope is more secure than one attached to the outside of the envelope. If an alkaline reserve sheet is used, it can be labeled.
It is important to remember that encapsulation, like any conservation technique, should not be applied to every document. The decision to use this strategy to preserve documents should be a matter of informed judgment, weighing the need to support or protect the document against the possibility that chemical deterioration may occur at an increased rate.
Polyester film is strong, flexible, and relatively inert. If free from plasticizers, UV-inhibitors, dyes, and surface coatings, they will not interact with documents. The thickness of the film should be chosen for its ability to support the surface area of the object being encapsulated; large objects require thicker film.
Northeast Document Conservation Center. 1991. Encapsulation. Technical Leaflet: General Preservation. Andover, MA.