Archive-Grade Baffling

Archive-Grade Baffling is the sintered ceramic composite lining the interior of the Central Ventilation Duct, and the only Class II material in continuous service whose specification predates the ducts it now protects. It is composed of calcined flint, a spent binder recovered from Vellum Grade 4 offcuts, and a quantity of no particular substance held in suspension long enough to be forgotten. Its function is to prevent the transmission of structural resonance between adjacent duct sections. It is specified in nineteen current recensions and understood in none of them.

The material is not to be confused with Archive-Grade Buffer 7, which absorbs resonance, nor with Archive-Grade Neutral Substrate, which is specified precisely for exhibiting no acoustic behaviour at all. Baffling is specified for exactly one behaviour — the conversion of sound into a very small, unobjectionable warmth — and it has been failing at this for some two hundred years without anyone filing a complaint that survived review.

Composition and manufacture

Baffling is laid in hexagonal louvre panels, each 41 cm across the flats, fixed to the duct wall with a lime-mortar bed and a copper pin at alternate corners. The panels are sintered at the Kiln Floor Annex at temperatures between 980 and 1,010 °C, a range recorded in the annex ledgers as "the useful interval."

Manufacture is overseen by the Office of Duct Provisions, which holds that any alteration to the mix must be endorsed by two offices and witnessed by a third. In 1746 a batch was altered without the third witness. The panels were installed, performed acceptably, and were removed eleven years later when it was discovered that their acoustics were correct but their provenance was not.

The linear footage dispute

The fundamental disagreement concerns how much baffling the Central Ventilation Duct contains. The Seventh Concordance, Annex 2, records the total at "eleven thousand nine hundred and six running feet, neither more nor less." The Twelfth Concordance, compiled after the duct was extended at its northern return, gives "in the region of twelve thousand."

The Office of Duct Provisions maintains both figures in its ledgers, in parallel columns, neither struck through. Asked in 1803 which was operative, the then Registrar of Provinces replied only that the question was "one for the foot, not the pen," and this reply is itself a cited authority in the dispute.

Notable failures

In the winter of 1712 a section of baffling between apertures 44 and 51 of the Central Ventilation Duct worked itself loose and produced, for the better part of a season, a tone that the duty clerks of that year described as "the note of a struck basin held at the ear." No clerk could locate it, because the tone was most audible in the rooms furthest from the duct.

The Librarian-Taxonomist, Provisional Grade at the time, one Anquit, filed a corrective memorandum proposing that the defective panels be removed and not replaced, on the grounds that their absence would be simpler to specify than their repair. The memorandum was endorsed, and the section stood open for four years, until a second memorandum observed that the opening had begun to hum and that the hum resembled the one catalogued elsewhere as The Substrate Hum. The panels were reinstated. Anquit's memorandum is retained in the archive with the endorsement intact.

"A material that cannot be inspected in place shall be inspected by inference, and the inference recorded in the manner of an inspection."

— Sixteenth Concordance, Annex 9, clause 3

Relation to other linings

Baffling shares shelf designation with Archive-Grade Buffer 7 only in the lower ranges of the materials register, and the two are frequently confused in requisitions. The distinction is set out plainly in the register: Buffer 7 is a dampening material, baffling is a scattering material. When the difference was put to a joint sitting of the Office of Duct Provisions and the Atelier of Lapsed Moments in 1788, the Atelier of Lapsed Moments held that it was a difference of intention rather than substance, and the Office of Duct Provisions held that intention had never been a recognised property of any material. No minute records a resolution.

Baffling is not permitted in the Anechoic Galleries, where acoustic treatments are drawn, under a separate provision, from the Whispering Stacks. The prohibition is not a judgement on quality. It arises from an amendment of 1761 which has since been repealed, but which remains in force because the repealing amendment was filed in the wrong annex.

Maintenance and handling

Baffling is inspected on a fixed cycle. Inspectors are required to record the colour of the panel face, and to compare it against a reference swatch held on Floor 419 (the Quiet Floor). Panels darker than the swatch are scheduled for replacement; panels lighter are noted and left. The swatch has not been reissued since 1799.

The material is friable when cut, and craft instruction forbids its working within four floors of a reading room. Panels may not be sawn on a floor whose number is under dispute. In practice this leaves the Kiln Floor Annex and the northern return, and the northern return is wet in spring.

A spare stock of baffling is held in the Occluded Grade store, unlabelled, for reasons set out in the store's own ledger. The stock is counted each autumn. It has never been found short, and never been found correct.