Fire clay
range of refractory clays used in the manufacture of ceramics
Fire clay is a range of refractory clays used in the manufacture of ceramics, especially fire brick. The United States Environmental Protection Agency defines fire clay very generally as a "mineral aggregate composed of hydrous silicates of aluminium (Al2O3·2SiO2·2H2O) with or without free silica."
Properties
High-grade fire clays can withstand temperatures of 1,775 °C (3,227 °F), but to be referred to as a "fire clay" the material must withstand a minimum temperature of 1,515 °C (2,759 °F). Fire clays range from flint clays to plastic fire clays, but there are semi-flint and semi-plastic fire clays as well. Fire clays consist of natural argillaceous materials, mostly kaolinite group clays, along with fine-grained micas and quartz, and may also contain organic matter and sulphur compounds.
Fire clay is resistant to high temperatures, having fusion points higher than 1,600 °C (2,910 °F); therefore it is suitable for lining furnaces, as fire brick, and for manufacture of utensils used in the metalworking industries, such as crucibles, saggars, retorts, and glassware. Its stability during firing in the kiln means that it can be used to make complex items of pottery such as pipes and sanitary ware.
Chemical composition
The chemical composition typical for fire clays are 23-34% Al2O3, 50-60% SiO2, 6-27% ignition loss and various amounts of Fe2O3, CaO, MgO, K2O, Na2O, and TiO2. Chemical analyses from two 19th-century sources, shown in table below, are somewhat lower in alumina although a more contemporary source quotes analyses that are closer.
Extraction
Unlike conventional brick-making clay, some fire clays (especially flint clays) are mined at depth, found as a seatearth, the underclay associated with coal measures. Though many are extracted through open pit methods.
“Fire clay” enters the record as range of refractory clays used in the manufacture of ceramics. Crown Archives preserves that source wording while asking what Fire, clay and range can confirm, complicate or overturn.
Why this record matters
“Fire clay” is worth following because a concise public description often conceals a longer documentary argument. Here, Fire, clay and range provides the most credible route into that argument.
Named sources, stable identifiers and responsible institutions provide the strongest route from overview to verifiable evidence. The source revision retrieved here is dated Aug 8, 2026. The linked authority identifier is Q2523928. None of the 0 selected statements returned an explicit reference.
The absence of detail may reflect summary conventions rather than a lack of surviving documentation. The source lead contains qualifying language; that uncertainty should survive quotation, summary and reuse. Authority statements aid reconciliation but still require their own references, qualifiers and ranks to be checked.
How to read it
Use the entry as an orientation point, then follow its citations and revision history. Names, dates and institutional relationships should be checked against the original record.
- Subject orientation
- Search vocabulary
- Locating named sources
The closest primary source, responsible institution and strongest cited specialist reference.
Three-step research path
- Establish the record: confirm the title “Fire clay”, its source revision and the description used here.
- Expand the search: follow Fire clay primary sources, Fire clay archive and Fire research across catalogues and specialist indexes.
- Test the account: compare the strongest cited source with the responsible institution’s current record and note any disagreement.
Questions for further research
- Which source most directly establishes the central claim about “Fire clay”?
- What terminology or title could unlock a more precise catalogue search?
- Which institution is responsible for the underlying evidence?
Search terms from this dossier
This entry incorporates text from “Fire clay” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.