Steatite C-221
Grade
ASTM C773 L-5 · ISO IEC 60672 Type C-221 · also Magnesium Silicate
C-221 = low-loss steatite
Pick when RF/VHF loss matters: alkali content is minimised so the 1 MHz loss tangent is roughly half that of C-220.
Values for the C-221 condition
Other Steatite tempers and conditions
Steatite is listed in 3 conditions; see the grade overview at Steatite.
Compare all 3 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Porous steatite | Pick for wire-wound resistor cores, bobbins and burner parts where open porosity aids cement bonding and thermal-shock survival, and strength is secondary. | 20 | 400 HV | 1 W/m·K | ||
As FiredDefault dense vitrified steatite (ASTM D116 C-220) | Standard dense, zero-porosity steatite as fired — general-purpose low-cost HF insulator and heater/terminal component. | 60 | 650 HV | 3 W/m·K | ||
C-221This page Low-loss steatite | Pick when RF/VHF loss matters: alkali content is minimised so the 1 MHz loss tangent is roughly half that of C-220. | 680 HV | 3 W/m·K |
What is Steatite C-221?
Steatite C-221 is Steatite in the C-221 grade — pick when RF/VHF loss matters: alkali content is minimised so the 1 MHz loss tangent is roughly half that of C-220. Steatite C-221 has a tensile strength of 60 MPa (8.7 ksi), weaker than 63% of ceramic grades. Elongation at break is 0% and the elastic modulus is 105 GPa (15.2 Msi). Steatite C-221 has a density of 2.8 g/cm³ (0.101 lb/in³), lighter than 80% of ceramic grades. It melts at 1,450°C (2,642 °F). As Fired is the default condition FabDigit quotes; C-221 is available on request or by drawing note.
Advantages
- Low embodied carbon — 1.9 kg CO₂/kg, better than 94% of ceramic grades
- Lightweight — 2.8 g/cm³ (0.101 lb/in³), better than 80% of ceramic grades
- Good corrosion resistance — 87/100, better than 75% of ceramic grades
Limitations
- Poor heat conductor — 2.6 W/m·K (1.5 BTU/hr·ft·°F), worse than 80% of ceramic grades
- Hard to polish — 60/100, worse than 79% of ceramic grades
Steatite C-221 properties
Typical room-temperature values for Steatite C-221; 12 are specific to this condition. Other listed values are Steatite reference data and may not represent this condition. Bars are the position among ceramic grades.
Physical3
Steatite C-221 has a density of 2.8 g/cm³ (0.101 lb/in³), lighter than 80% of ceramic grades. It melts at 1,450°C (2,642 °F).
Mechanical12
Steatite C-221 has a tensile strength of 60 MPa (8.7 ksi), weaker than 63% of ceramic grades. Elongation at break is 0% and the elastic modulus is 105 GPa (15.2 Msi).
Thermal5
Steatite C-221 is rated for continuous service to 1,050°C (1,922 °F). It conducts heat at 2.6 W/m·K (1.5 BTU/hr·ft·°F), worse than 80% of ceramic grades. Thermal expansion is 8 µm/m·K (4.44 µin/in·°F).
Electrical5
Steatite C-221 is an electrical insulator with a dielectric strength of 12 kV/mm (304.8 V/mil) and a resistivity of 1×10¹³ Ω·m.
Chemical & Environmental2
Corrosion resistance is excellent (87/100), better than 75% of ceramic grades.
Sustainability4
Producing a kilogram of Steatite C-221 takes about 27 MJ of energy and emits 1.9 kg of CO₂ — less than 95% of ceramic grades. Typical recycled content is 2%.
Manufacturability3
Steatite C-221's machinability is not recommended (8/100, better than 69% of ceramic grades).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with Steatite C-221
Is Steatite easy to machine?
Machine in the green/bisque state (drill, turn, mill dry with carbide) allowing ~10–14 % firing shrinkage; fired parts require diamond grinding, lapping or laser/ultrasonic drilling.
Can Steatite be welded?
Not weldable — join by glass frit, ceramic cement, active-metal or Mo-Mn metallizing plus brazing, or mechanical clamping with compliant gaskets.
Can Steatite be formed, bent or molded?
Dry-pressed, isostatically pressed, extruded (tubes/rods) or injection/wet-pressed from talc–clay–flux batches, then fired at 1250–1350 °C; the vitrification window is narrow (~±20 °C), so temperature uniformity controls warpage and porosity.
What surface finishes work on Steatite?
Fired surface is naturally smooth and can be glazed for tracking resistance; ground/lapped for flatness, and Mo-Mn or thick-film Ag metallizing is used for solderable/brazable pads.
Steatite chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| SiO2from talc + clay | 55 – 65 | 61 |
| MgOprotoenstatite former | 25 – 32 | 29 |
| Al2O3clay addition | 2 – 8 | 5 |
| BaOflux; BaCO3 used in low-loss C-221 instead of alkali feldspar | 0 – 6 | 3 |
| CaO | 0 – 2 | 0.7 |
| Na2O+K2Okept <0.5 % in low-loss C-221 | 0 – 2 | 0.5 |
| Fe2O3impurity | 0 – 1 | 0.3 |
Steatite C-221 — frequently asked
What is Steatite C-221 used for?
Steatite C-221 is typically used for electrical insulators and bushings, heater coil supports, terminal blocks and fuse bodies, thermostat and switch insulating parts, lamp holders and sockets and extruded insulating tubes and beads. In short: low-cost electrical porcelain.
What is the tensile strength of Steatite C-221?
Steatite C-221 has a typical tensile strength of 60 MPa (8.7 ksi) — weaker than 63% of ceramic grades. Strength varies by condition: see the 3 listed tempers.
Is Steatite C-221 easy to machine?
Not especially — machinability is rated not recommended (8/100, better than 69% of ceramic grades). Machine in the green/bisque state (drill, turn, mill dry with carbide) allowing ~10–14 % firing shrinkage; fired parts require diamond grinding, lapping or laser/ultrasonic drilling.
Can Steatite C-221 be welded?
Not weldable — join by glass frit, ceramic cement, active-metal or Mo-Mn metallizing plus brazing, or mechanical clamping with compliant gaskets.
What surface finishes work on Steatite C-221?
Fired surface is naturally smooth and can be glazed for tracking resistance; ground/lapped for flatness, and Mo-Mn or thick-film Ag metallizing is used for solderable/brazable pads.
Can Steatite C-221 be formed, bent or molded?
Dry-pressed, isostatically pressed, extruded (tubes/rods) or injection/wet-pressed from talc–clay–flux batches, then fired at 1250–1350 °C; the vitrification window is narrow (~±20 °C), so temperature uniformity controls warpage and porosity.
What is the maximum service temperature of Steatite C-221?
Steatite C-221 is rated for continuous use to about 1,050°C (1,922 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
Sources
- ASTM D116 — Standard Test Methods for Vitrified Ceramic Materials for Electrical Applications (grades C-220, C-221) — ASTM International (2016)
- ASM Engineered Materials Handbook, Vol. 4: Ceramics and Glasses — ASM International (1991)
- Steatite technical ceramic property sheets — CoorsTek / CeramTec / Kyocera (2023)
- Technical ceramics property tables (steatite, magnesium silicate) — MatWeb (2024)
Property data is compiled from published supplier and standards handbooks and normalised for comparison. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.