EARTHENWARE CLAY

Miscellaneous

Family: Clay Other
Region: None
Mined At: Unspecified
Raw Mineral: Yes
Generic: No

Notes

(Richard Willis)

Traditionally, earthenware clays have been natural clays (buff, red, terra-cotta, etc.) which mature in firings under 1100ºC to a light-weight porous fabric. For the most part, earthenware clays used for cooking and serving tableware, cereal storage, liquid storage, and so forth by cultures since Neolithic times rarely fire over 900ºC. Notable exceptions created the faience and delft wares, among others, firing between 900ºC and 1150ºC.
Earthenware clays, whether natural or composed, typically contain much less feldspar, and are fired at much lower temperatures, than are usual for stonewares and porcelains; while, on the other hand, higher firings and greater portions of feldspar (and/or “talc”, “crete”, etc.) often distinguish fine pottery earthenwares from other “common brick-clay” wares. Earthenware glazes tend to be fluxed principally by lead and/or boron rather than by alkalis, as are stoneware and porcel ain glazes, even though there may well be alkalis abundantly present in the over-all formulation of earthenware glazes — such as the sodium, potassium, calcium, etc., integral to the clay and/or feldspar additions being used. Since feldspars makeup some 51% of the Earth’s crust it is rare that any given clay will not contain substantial traces; and feldspathic presence — as incidental or necessary — is often the principal manufactural factor determining vitrifiability and t hereby distinguishing stonewares and porcelains from earthenwares.
Glazes suitable for faience, maiolica and delft, to name a few of the better known tin-glazed earthenwares, are quite suitable for earthenware clays and firing conditions in general.
Common firing ranges are 650-900ºC for “common” wares and 1050-1150ºC for “fine” wares. Recipe bases are usually made up of lead and/or boron, pegmatite and quartz.
A natural, “surface” or “mined”, earthenware clay is typically analyzable at around the following: CaO 41.0, SiO₂17.1, Al₂O₃ 7.2, K₂O 0.5, Na₂O 0.3, Fe₂O₃ 0.3, TiO₂ 0.2, MgO 0.01, FeO/MnO traces; and a high-fire (nearing 1200ºC ) earthenware (such as those used for creamware and faience) can be made by a composition of Cornwall stone (or any pegmatite) 3-12%, flint or quartz 35-55%, and a low temperature common porous calcareous clay (such as represented by the above analysis) 40-55%.
clay and glaze recipes in dry-weight parts (further recipes are listed under titles for colors and types)
650-900ºC (buff “common pottery” clays)
— adobe or marl clay 40, red clay 40, sand 20
— common calcareous and/or ferrous clay 90, sand 10
— red clay 80, yellow “ochre” clay 20
— common red “brick” clay 80, sand 20
— red clay 68, silex or cristobalite 32
— red clay 70, fire clay 20, quartz or sand 10
—— white engobes: fire clay 50, pegmatite 35, kaolin 10; fire clay 30, kaolin 30, nepheline 10, lead 5; fire clay 40, kaolin 25, nepheline 20, crete 10
—— transparent base glazes: minium 55, silica 45; hydroboracite 50, quartz 50; borax 55, sand 40, wood ash 5
—— opaque base glazes: lead 70, nepheline 30; hydroboracite 80, nepheline 20; lead or hydroboracite 70, sand 20, nepheline or pegmatite 7, wood ash 3
—— opaque-white base: 75 parts white lead to 25 parts tin, then add 89 parts of this “base” to 7 crete, 5 flor-spar, NaCl 17.5, borax 38, kaolin 13, and quartz 48 — calcinate at 650ºC
white “fine pottery” clays
(“fire clay” = any refractory clay high in calcium and/or alumina yet low in iron and silica that fires to a white color)
900ºC fire clay 80, quartz or sand 20, crete 50
940ºC fire clay 50, crete 30, nepheline or pegmatite 20
950-1100ºC fire clay 40, silex or cristobalite 40, crete 20; fire clay 60/65, silica 60/65, pegmatite 5/10
1100ºC fire clay 70, silica 30; fire clay 22, pegmatite 23, fine sand 55
1100-1200ºC fire clay 36/25/30, pegmatite 24/31/40, fine sand 27/39/16, nepheline 13/5/14
transparent “cover” glazes for the fine pottery clays
800-1000ºC: lead or borax 80, pegmatite 15, sand 5
1000ºC base: nepheline 45, quartz 15, lead or hydroboracite 10, crete 15, sod. nitrate 10, nepheline 50 — (recipe = base 100, ceruse 5, nepheline 5)
1050ºC base: hydroboracite 40, nepheline 35, quartz 10, crete 17 — (recipe = base 50, hydroboracite or ceruse 20, nepheline 20, silica 10)
1080-1140ºC base: lead or hydroboracite 16, quartz or sand 10, crete 3, nepheline 3 — (recipe = base 75, nepheline 25)
1100ºC base: nepheline 55, silica or sand 10, crete 8, minium or hydroboracite 30 — (recipe = base 80, nepheline 10, ceruse or hydroboracite 10
1180ºC base: hydroboracite 10, nepheline 12, crete 7, sand 10 — (recipe: base 90, nepheline 10)
(Note: base glazes always work better when fritted prior to recipe composing, particularly where there are soluble ingredients, but if mixed well while sifting very finely and used while fresh (before sedimentation/crystallization of solubles) then fritting may be neglected)

Authors

Richard Willis (Owner)

XML

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<material name="EARTHENWARE CLAY" descrip="" generic="0" rawmineral="1" searchkey="" loi="0.00">
<families>
<family name="Clay Other"/>
</families>
<notes>
<note>Traditionally, earthenware clays have been natural clays (buff, red, terra-cotta, etc.) which mature in firings under 1100&ordm;C to a light-weight porous fabric. For the most part, earthenware clays used for cooking and serving tableware, cereal storage, liquid storage, and so forth by cultures since Neolithic times rarely fire over 900&ordm;C. Notable exceptions created the faience and delft wares, among others, firing between 900&ordm;C and 1150&ordm;C. 
Earthenware clays, whether natural or composed, typically contain much less feldspar, and are fired at much lower temperatures, than are usual for stonewares and porcelains; while, on the other hand, higher firings and greater portions of feldspar (and/or &#147;talc&#148;, &#147;crete&#148;, etc.) often distinguish fine pottery earthenwares from other &#147;common brick-clay&#148; wares. Earthenware glazes tend to be fluxed principally by lead and/or boron rather than by alkalis, as are stoneware and porcel
ain glazes, even though there may well be alkalis abundantly present in the over-all formulation of earthenware glazes &#151; such as the sodium, potassium, calcium, etc., integral to the clay and/or feldspar additions being used. Since feldspars makeup some 51% of the Earth&#146;s crust it is rare that any given clay will not contain substantial traces; and feldspathic presence &#151; as incidental or necessary &#151; is often the principal manufactural factor determining vitrifiability and t
hereby distinguishing stonewares and porcelains from earthenwares. 
Glazes suitable for faience, maiolica and delft, to name a few of the better known tin-glazed earthenwares, are quite suitable for earthenware clays and firing conditions in general. 
Common firing ranges are 650-900&ordm;C for &#147;common&#148; wares and 1050-1150&ordm;C for &#147;fine&#148; wares. Recipe bases are usually made up of lead and/or boron, pegmatite and quartz. 
A natural, &#147;surface&#148; or &#147;mined&#148;, earthenware clay is typically analyzable at around the following: CaO 41.0, SiO2 17.1, Al2O3 7.2, K2O 0.5, Na2O 0.3, Fe2O3 0.3, TiO2 0.2, MgO 0.01, FeO/MnO traces; and a high-fire (nearing 1200&ordm;C
) earthenware (such as those used for creamware and faience) can be made by a composition of Cornwall stone (or any pegmatite) 3-12%, flint or quartz 35-55%, and a low temperature common porous calcareous clay (such as represented by the above analysis) 40-55%. 
clay and glaze recipes in dry-weight parts (further recipes are listed under titles for colors and types) 
650-900&ordm;C (buff &#147;common pottery&#148; clays) 
&#151; adobe or marl clay 40, red clay 40, sand 20 
&#151; common calcareous and/or ferrous clay 90, sand 10 
&#151; red clay 80, yellow &#147;ochre&#148; clay 20 
&#151; common red &#147;brick&#148; clay 80, sand 20 
&#151; red clay 68, silex or cristobalite 32 
&#151; red clay 70, fire clay 20, quartz or sand 10 
&#151;&#151; white engobes: fire clay 50, pegmatite 35, kaolin 10; fire clay 30, kaolin 30, nepheline 10, lead 5; fire clay 40, kaolin 25, nepheline 20, crete 10 
&#151;&#151; transparent base glazes: minium 55, silica 45; hydroboracite 50, quartz 50; borax 55, sand 40, wood ash 5 
&#151;&#151; opaque base glazes: lead 70, nepheline 30; hydroboracite 80, nepheline 20; lead or hydroboracite 70, sand 20, nepheline or pegmatite 7, wood ash 3 
&#151;&#151; opaque-white base: 75 parts white lead to 25 parts tin, then add 89 parts of this &#147;base&#148; to 7 crete, 5 flor-spar, NaCl 17.5, borax 38, kaolin 13, and quartz 48 &#151; calcinate at 650&ordm;C 
 white &#147;fine pottery&#148; clays 
(&#147;fire clay&#148; = any refractory clay high in calcium and/or alumina yet low in iron and silica that fires to a white color) 
900&ordm;C fire clay 80, quartz or sand 20, crete 50 
940&ordm;C fire clay 50, crete 30, nepheline or pegmatite 20 
950-1100&ordm;C fire clay 40, silex or cristobalite 40, crete 20; fire clay 60/65, silica 60/65, pegmatite 5/10 
1100&ordm;C fire clay 70, silica 30; fire clay 22, pegmatite 23, fine sand 55 
1100-1200&ordm;C fire clay 36/25/30, pegmatite 24/31/40, fine sand 27/39/16, nepheline 13/5/14 
transparent &#147;cover&#148; glazes for the fine pottery clays 
800-1000&ordm;C: lead or borax 80, pegmatite 15, sand 5 
1000&ordm;C base: nepheline 45, quartz 15, lead or hydroboracite 10, crete 15, sod. nitrate 10, nepheline 50 &#151; (recipe = base 100, ceruse 5, nepheline 5) 
1050&ordm;C base: hydroboracite 40, nepheline 35, quartz 10, crete 17 &#151; (recipe = base 50, hydroboracite or ceruse 20, nepheline 20, silica 10) 
1080-1140&ordm;C base: lead or hydroboracite 16, quartz or sand 10, crete 3, nepheline 3 &#151; (recipe = base 75, nepheline 25) 
1100&ordm;C base: nepheline 55, silica or sand 10, crete 8, minium or hydroboracite 30 &#151; (recipe = base 80, nepheline 10, ceruse or hydroboracite 10 
1180&ordm;C base: hydroboracite 10, nepheline 12, crete 7, sand 10 &#151; (recipe: base 90, nepheline 10) 
(Note: base glazes always work better when fritted prior to recipe composing, particularly where there are soluble ingredients, but if mixed well while sifting very finely and used while fresh (before sedimentation/crystallization of solubles) then fritting may be neglected) 

</note>
</notes>
</material>

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EARTHENWARE CLAY

Miscellaneous

Notes

(Richard Willis)

Authors

XML