Copper (metal flake)

Copper (CAS 7440-50-8) in anti-seize compounds appears as fine metal flake — particles ranging from a few microns to tens of microns in diameter, suspended in a petroleum-grease carrier. The copper flake provides the core function of anti-seize: at the thread interface under load and heat, the copper particles deform plastically and fill microscopic surface irregularities, creating a lubricating barrier that prevents metal-to-metal contact (galling) and maintains the ability to disassemble fasteners cleanly after years of thermal cycling.

Copper anti-seize is the most widely used formulation in automotive service because copper withstands temperatures up to approximately 1800°F (982°C) without breaking down — covering spark plug threads, exhaust manifold studs, brake-caliper hardware, and most passenger-vehicle exhaust applications. The limitation is compatibility with stainless steel at sustained high heat: copper can cause stress-corrosion cracking on stainless steel fasteners exposed to prolonged thermal cycling above approximately 400°F (204°C), making nickel-based anti-seize the correct choice for stainless exhaust applications.

The primary environmental concern with copper metal flake is aquatic toxicity. Copper ions in water are acutely toxic to aquatic invertebrates (NOEC for Daphnia magna: less than 0.01 mg/L) and fish at ppb-level concentrations. This toxicity is the basis for copper's UN 3082 marine-pollutant transport classification and for its inclusion in virtually every copper anti-seize product's aquatic-toxicity deduction under environmental scoring.

Regulatory status

• Not California Prop 65 listed: no entry for copper or any copper compound on the OEHHA list (checked against the repo's data/prop65/oehha-list.json, plus a live cross-check of p65warnings.ca.gov's chemical index, which also carries no copper entry). OEHHA maintains a background informational page on "Copper and Copper Compounds," which documents that the substance has been considered/reviewed, not that it is listed; a considered-but-not-listed chemical does not carry the warning.
• Not an NTP Report on Carcinogens listing: copper does not appear among the substances in the NTP's cumulative Report on Carcinogens (256 listings as of the 15th edition).
• A prior version of this file asserted prop65_listed: true and described copper as an "NTP known carcinogen." Neither claim is correct; both are corrected here.

Sources

• OEHHA Proposition 65 list cross-check: data/prop65/oehha-list.json (986 entries, published 2025-12-05), no entry by name "copper" or CAS 7440-50-8; corroborated against p65warnings.ca.gov/chemicals (no copper-related entry found).
• NTP Report on Carcinogens (15th edition, 256 listed substances): no copper entry.
• Blast-radius check: two consuming products. content/products/permatex/permatex-09128-copper-anti-seize/index.md already resolves prop65_warning: false via an independent SDS §15 affirmative-negative (amorphous silica asterisk-exempted; the product page explicitly notes the Amazon warning flag looked stale and was overridden), so this correction does not change its published value. content/products/loctite/loctite-lb-8008-c5-a-anti-seize/index.md resolves prop65_warning: true, but its own ingredients[] also carries quartz-crystalline-silica (genuinely OEHHA listed, see that ingredient's own Sources section), which independently drives the same true resolution, so this correction does not change its published value either. That product's rationale text currently cites "copper compounds + quartz/crystalline silica" as joint Prop 65 drivers; the "copper compounds" half of that citation is now stale and should be corrected on a future regen of that product (out of scope here; product files are not owned by this pass).