BPG Drop Spine Box

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This type of enclosure is a common choice for rehousing books. It is also called a cloth-covered clamshell box or double-tray clam shell box. It is constructed from three parts, the small tray, the large tray, and the case.

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Basic Instructions

General Information

  • These instructions are modified from my training at the North Bennet Street School 2004-2006.
  • The instructions are for a drop spine box made with a buckram weight bookcloth and .098 binder's board. Other combinations of materials will require minor adjustments in the allowances.


  1. Measure the length, width and depth of the book, adding one board thickness to the depth and an additional 1/16 inch to the length to allow for the turn-ins of covering material.
  2. On the board shear, cut a strip from a full sheet of binder's board that is one inch larger than the length of the book being boxed (e.g. if the book is 5 1/2 x 8 inches, cut a 9 inch strip).
  3. Square one corner of the strip and cut three walls for the small tray to the correct depth.
  4. Add 1 1/2 board thicknesses and cut the three walls for the large tray.
  5. Next cut the width of the base of the small tray.
  6. Add one board thickness and cut the head and tail walls for the small tray.
  7. Add another 1/2 board thickness and cut the width of the base for the large tray.
  8. Add one board thickness and cut the head and tail walls for the large tray.
  9. Cut the length of the base of the small tray and the fore-edge wall of the small tray.
  10. Add about three boards thicknesses and cut the length of the base of the large tray and the fore-edge wall of the large tray.
  11. Assemble the small tray first. Check the fit of the tray at this point! If there is any problem it will be easier to correct at this early stage than after you've begun covering the trays.
  12. Cover the trays as shown in the following illustrations.
  13. For the case, cut two pieces of binder's board the length of the trays plus 2 or 3 board thicknesses, by the width of the assembled trays plus 1 or 1 1/2 board thicknesses. Cut a spine piece the same length as the boards by the exact depth of the large tray. Cover the case, leaving an allowance for the joints that is the thickness of the board you've used for the case plus one thickness of 4-ply mat board.
  14. To assemble the box glue out the bottom of the small tray with straight PVA and place inside the large tray. Place both trays in position and close the case checking that the squares are where you want them and the box stands upright evenly. Open the case and remove the large tray being careful not to move the small tray. Press the small tray under heavy pressure for at least 20 minutes. Glue out the bottom of the large tray, place it on top of the small tray and bring the case over. Check the fit and then carefully pull the two trays apart without moving the large tray. Press the large tray under heavy pressure for at least 20 minutes and remove. Close the completed box and leave to rest under weight over night.

Variations on the Theme

An additional strip of cloth is usually added to the inside spine of the case. This cloth extends either under the trays, or slightly over them.

The base of each tray can be lined with archival paper. This paper can fold around the spine edge of each tray, or be simply adhered to the floor of the tray.

Some conservators choose to cut the case to the exact length of the trays, rather than leaving a square at head and tail. This allows the box to sit flush on the shelf. A slight square at the fore edge is necessary to allow the box to be opened[1].

The page Drop Spine Box with Interior Cradle presents several varieties of display-friendly boxes with collapsible book supports.

Foam supports inside the box can accommodate non-rectangular objects.

Decorative Variations

The entire box, or just the spine, can be covered with leather and tooled.

Variations that Apply Additional Pressure

Several variations on the drop spine box have been developed to apply additional pressure to the book. This is particularly useful for stiff-board vellum bindings that tend to yawn open at the fore edge. Some options:

  • A fixed shelf 2-3" wide, is added at the fore edge side of the lower tray. The book slides under this shelf as it is placed in the box.
  • A hinged flap, the width and length of the book, is added to the fore edge side of the lower tray. This lifts out of the way when the box is open, but compresses the book when the box is closed.
  • Velcro straps or cloth ties are added to the fore edge side of both trays. They securely fasten the box closed.

In January 2016, Henry Hebert wrote a blog post, "Hold me closer...protective enclosures" that discusses various methods of building pressure boxes for vellum bindings.[2]

Adhesives Used

Some conservators have chosen to substitute wheat starch paste or methyl cellulose for some or all of the PVA because of concerns with PVA off-gassing within the confined enclosure. Other conservators continue to use PVA, but let the boxes air out for several days to allow most of the acetic acid to disperse[3].


  1. Carlson, Lage, John Bertonaschi, Margot Healey, Lynn Kidder, Nancy Lev, Bob Muens, Carol Paulson, and Carrie Beyer. 1994. "Double-Tray Clam Shell Box". Chapter 5 from Boxes for the Protection of Books: Their Design and Construction. Available online though Google Books. Accessed January 1, 2016.
  2. Hebert, Henry. 2016. "Hold me closer...protective enclosures" January 15, 2016 Blog post for Preservation Underground. Accessed March 9, 2016.
  3. Brewer, Allison. 2013. "Acetic Off Gassing in Clamshell Enclosures". Poster Presentation at the 2013 American Institute for Conservation Annual Meeting, Indianapolis.
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