A gasket chemical-compatibility chart is a matrix: chemicals (or chemical families) run down the rows, gasket material families run across the columns, and each cell carries a single-letter or symbol rating that summarises how that material behaves in contact with that fluid [S1]. The first move when reading one is to locate your service chemical in the leftmost column, not to scan material columns first, because the fluid dictates which columns are even worth comparing.
Most published charts, including the Durlon, RubberFab, Hennig, Cole-Parmer, and CASRAI references, use the same conventions: A for Acceptable, C for Conditional (or "use with caution"), NS/U for Not Suitable, and a dash or blank for "no data" [S1][S2][S3][S7][S9]. Treat the legend as a contract: every cell below it inherits that legend's meaning, and a cell with no rating is not the same as a cell marked A.
Decoding the cell codes: A, C, NS, and "no data"
The A rating on the Durlon chart means the material is "generally suitable" for that chemical service, while C means "Caution: Dependent on Conditions" and explicitly factors in concentration, temperature, and pressure before a decision can be made [S1]. Blaylock Gasket's guide repeats the same hierarchy: A is acceptable, C is "use it with caution," and other symbols are spelled out in the chart's own legend [S3]. A "C" is not a rejection; it is a flag that the chart is refusing to make the call without your operating data.
NS (or U, or a red square in colour-coded versions such as the Ansell guide) means the material is not recommended for that service; using it anyway trades a known failure mode for a paperwork problem [S1][S6]. A blank or dash means the manufacturer has not published a rating, which is a soft NS, not an implied A: an unrated PTFE/acid combination on a vendor chart should be treated as "ask the vendor," not as "safe by default" [S1][S2].
Reading across material families: non-asbestos sheet, PTFE, graphite, elastomers
Typical chart columns cover four to five material families, and each one has a known chemical personality. Compressed non-asbestos fibre (CNAF) sheet is the general-purpose column, suitable for water, oils, steam, and mild chemicals [S1]. PTFE columns show the broadest chemical coverage, including strong acids, solvents, and caustics, which is why PTFE replaced asbestos in most aggressive chemical service from the mid-20th century onward [S1][S5].
Flexible graphite columns dominate high-temperature and thermal-cycling services, including steam and hydrocarbons above the PTFE ceiling [S1][S5]. Elastomer columns (EPDM, Viton/FKM, Buna-N/NBR, neoprene, silicone) sit beside those and are rated against specific fluid families: EPDM for caustics and steam, Viton for fuels and aggressive solvents, Buna-N for oils and hydrocarbons, and so on [S3][S5]. A standard read pattern is to find the chemical row, then scan A ratings down that row; if more than one material rates A, move to temperature and pressure sub-tables to break the tie [S1][S8].
Why the chart is a starting point, not a purchase order
Durlon's own guidance is explicit: "Chemical compatibility charts are general guides only. Final gasket selection should consider the complete application, including chemical concentration, pressure, temperature, flange condition, cycling, media purity, and applicable safety requirements" [S1]. Dobson Gasket repeats the same warning for corrosive service, adding that concentration, temperature swings, and mechanical load can flip a chart's A into a field failure [S5]. The chart reduces a 50-material catalogue to two or three candidates; the engineer's job is to disqualify the rest with the four variables the chart does not cover.
For chemicals at elevated temperature, the "C" rating in a room-temperature chart almost always means "de-rate the temperature and recheck." For batch processes with cleaning agents, peroxides, or caustics in sequence, a chart only sees the static pair, not the transient pair, and pharmaceutical and food-service ratings add FDA and NSF 61 requirements on top of the chemistry question [S3][S5]. Where the application involves hydrocarbons with H₂S, NACE MR0175 limits the elastomer and metallic gasket menu independently of the chart's chemistry result.
Comparing common gasket families on four decision criteria
Side-by-side against a typical mixed-chemical plant service, the four workhorse families sort like this. CNAF scores well on cost and availability, mid-pack on temperature (usually capped below 400 degC) and weak on aggressive chemicals [S1][S3]. PTFE scores highest on chemical compatibility, including strong acids and solvents, but loses on mechanical creep and cold-flow, which forces fillers (glass, graphite, bronze) or a restructured-PTFE form for high-pressure service [S1][S3][S5]. Flexible graphite leads on temperature (continuous service well above 500 degC in non-oxidising media) and thermal cycling, but requires oxidation inhibitors or inert barriers in strong oxidisers, and it is not rated for the broad chemical sweep that PTFE covers [S1][S5]. Elastomers (EPDM, Viton, Buna-N) are the cheapest and best-fitting soft gaskets, but each one is a specialist: EPDM fails in hydrocarbons, Viton fails in amines and hot water/steam above its rating, Buna-N fails in polar solvents, and the chart's letter grade is valid only inside that elastomer's narrow window [S3][S5].
A useful internal link for the broader chemistry context is the chemical material encyclopedia entry, which covers the underlying polymer and reagent families referenced on these charts. For users who need to confirm that a chosen gasket is also approved for a wetted system as a whole, the chemical reagent compatibility reference extends the read from gaskets to tubing, seals, and process equipment [S9]. The gasket selection fundamentals page is the natural back-reference for the non-chemistry half of the decision (flange type, stress, ASME B16.20 vs B16.21).
Concrete read-through: sulfuric acid, sodium hydroxide, and steam
Walking the chart on three real services makes the reading pattern obvious. Sulfuric acid at ambient concentration rates A on PTFE, C on flexible graphite (because of concentration and oxidising strength), and NS on most elastomers, which immediately eliminates the elastomer columns and forces the decision into PTFE versus CNAF-with-acid-resistant binder [S1][S5]. Sodium hydroxide (caustic) inverts the elastomer result: EPDM rates A where Buna-N and Viton rate NS, PTFE still rates A, and CNAF's rating is C pending temperature, since hot caustic attacks the binder system of generic CNAF [S1][S3].
Steam is the boundary case that breaks chart-only logic: PTFE rates A on water but loses continuous-rating status as temperature approaches its upper limit, flexible graphite and CNAF rate A through the full steam envelope, and EPDM rates A only up to its steam ceiling (commonly cited around 150 degC saturated, depending on grade), above which the cell flips to C or NS even though the chemistry is unchanged [S1][S3][S5]. That is why published guides treat temperature as a co-equal screening axis with chemistry, not a footnote [S8].
Who should and should not rely on a compatibility chart alone
Maintenance teams ordering a replacement gasket for a known service, and buyers standardising a bill of materials for a defined chemical list, are the chart's intended users, and an A-grade match from a recognised chart is defensible for those routine cases [S1][S7]. The chart is not a substitute for vendor data sheets on a new chemical, on a concentration that the chart does not list, on a service that combines two fluids in sequence, or on any Class I Div 1 / ATEX / IECEx hazardous-area application where the gasket choice is tied to a larger equipment certification [S1][S5].
Process engineers qualifying a new chemical service, or any application with thermal cycling above the chart's published test temperature, should treat the chart as a shortlist generator and then validate with a vendor's material data sheet, a published case history, or a bench soak test in the actual fluid at the actual temperature [S1][S5][S8]. Skipping that step is the most common way a "chart-approved" gasket fails in service, and it is also the gap that vendor technical-support lines exist to close.
Where the data still has gaps in 2026
Even the major published charts, Durlon, RubberFab, Hennig, Cole-Parmer, and CASRAI, still show blank or "no data" cells for less common fluids, for elevated-temperature/high-concentration combinations, and for newer chemistries that the chart authors have not tested [S1][S2][S4][S7][S9]. The CASRAI wetted-materials reference, refreshed in August 2026, explicitly extends the read from gaskets alone to the full wetted material set (gaskets, seals, tubing, process equipment) so that a gasket choice consistent with the chart is not silently undermined by an incompatible O-ring or hose downstream [S9]. Until vendors close those blanks, the rule is: trust the A, respect the C, refuse the NS, and treat the dash as a question to send back to the manufacturer.
Trackable signals for the next quarter: any 2026 update to the CASRAI wetted-materials matrix beyond the August 2026 release [S9]; new chemical rows added to the Durlon and RubberFab PDFs (their 2025 editions remain the current published versions as of the reference date) [S1][S2]; and any vendor revision that publishes temperature-concentration-pressure sub-grids inside what is currently a single A/C cell, which would be the most useful chart evolution for working engineers.
This topic is covered further in ISO 5294 inch-pitch timing pulley dimensions: spec, selection, and gotchas.