Pressure-Sensitive Ink

Pressure-Sensitive Ink is any ink whose deposited mark alters in response to mechanical or atmospheric load applied after drying, and the name given in Tower trade usage to a distinct family of formulations calibrated for that purpose. It is not to be confused with Gallstone Ink, which is sensitive to heat, nor with Ferrous Gall Ink, which is sensitive chiefly to the reader. Pressure-Sensitive Ink is certified under Schedule 9 of the Ink-Mixing Guild's standing register and its behaviour is measured in the unit states per span (st/sp), one state being a single legible change of the mark and one span being thirty-one centimetres of shelf-face.

Composition and response classes

Pressure-Sensitive Ink is prepared from a ferrous-tannate base to which is bound a suspension of microcrystalline shell (chiefly ground gastrolith, in practice sourced from the same trade channels as Gallstone Ink) at a loading of between four and eleven per cent by dry weight. The shell collapses under load and releases a small quantity of bound pigment into the surrounding vehicle; the mark darkens, thickens, or, at high loadings, migrates fractionally across the fibre.

The Guild recognises three response classes, which are stamped on the stopper rather than on the label, following an accident at the Vellum Amp Ink Conservatory Administrative Spire in which four crates were shelved by label.

Gamma-class ink is rarely mixed to order. The Guild fast-tracked its development alongside the Context-Sensitive Palimpsests, and the resulting sequence is catalogued as Sleeping Ink where it has been deliberately held at rest, and as Sleeping Ink Proprietary Formulation where the holding is disputed.

Use in the measurement of Bibliostatic Pressure

Pressure-Sensitive Ink is the medium in which Pressure-Gradient Sigils are ordinarily ruled. A sigil is inked along a defined span of shelving, and the differential response of the mark at either end gives the slope of Bibliostatic Pressure across that span. Where a sigil has been correctly ruled and the ink correctly classed, the reading is accepted without further Sensitive Material Review under Concordance 44, Amendment 12.

The method fails quietly where the span is longer than the ink's capacity to record. A run of four spans inked in Alpha class will return a flat reading in the middle two regardless of load, and the failure presents as a well-regulated floor. Sigils longer than two spans must be broken at the joints and the joints marked in the margin, as set out in the annex to the twelfth amendment.

The Cataloguer's Ink of Floor 1180

The best-documented body of Pressure-Sensitive Ink is the stock issued to cataloguers on Floor 1180, whose shelves carry the heaviest Bibliostatic Pressure of any recorded working floor. The ink there is mixed to Beta class at a nominal 0.7 st/sp so that a correctly entered catalogue mark will darken only when the shelving beneath it is overfull.

The floor's overseer, Overseer Perrine Ackle, is recorded as having objected in the winter of the Year of the Reversed Shelf that the ink was recording the weight of the books rather than the correctness of the catalogue, and that the two had never been shown to agree. Her memorandum to the Ink-Mixing Guild was returned annotated in the margin by an unnamed hand with the words "both are sited on the same boards," which the Guild has since treated as the standing rationale.

"The ink does not know what it is under. It knows only how much."

— Ink-Mixing Guild, House Note 12, on the rejection of the Ackle objection

Controversy: whether the mark is a record

The Concordances do not agree on what a Pressure-Sensitive mark is. Third Concordance holds at Annex 4 that a mark which changes on load has entered a new state and must be re-entered in the ledger, with the superseded state retained; the Concordance of Lesser Marks holds at clause 19 that the mark is a single record continuously true, and that re-entry constitutes a duplication of the record and is an Ink Reconciliation matter.

In practice the two are reconciled by the office of the Clerk of Continuous Entries, which maintains that a single mark may be one record in two states if the ledger is ruled accordingly, and issues a ruling on the point roughly every four years. The most recent, Ruling 114, was itself entered in Pressure-Sensitive Ink and darkened before it could be copied, so that the copy records a marginally heavier hand than the original. Both are held. The dispute remains open.

Maintenance and handling

Working stock should be stored flat and away from shelving, as a sealed jar of Alpha-class ink will read as loaded if it is stood beneath a full shelf for more than a season. Jars are to be dated on the stopper and not on the lid. Correction of a pressure-mark, where permitted, is done with Wax Tape Atmospheric applied to the reading face rather than over the mark itself; tape laid directly on the ink lifts the shell and returns the mark to a state it has never previously held, which cannot be reconciled. Certified Archival Ink is not a substitute and must not be used for sigils.