Silicate Ceramics and Machinable Glass-Ceramics
Silicate ceramics are the oldest technical ceramics — magnesium, aluminum and calcium silicates fired from clay-like bodies into steatite, cordierite, mullite and porcelain — plus the machinable glass-ceramics built on them, of which Macor (55 % mica in a borosilicate glass) is the reference. They are weaker than alumina, typically 60–140 MPa in bending, but they are cheap to press in volume, excellent low-loss insulators, and several of them can be turned, milled, drilled and tapped with carbide tooling to ±0.01 mm without any firing step.
Popular silicate ceramic grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Weldability | Best for |
|---|---|---|---|---|---|---|
| Macor | 95 | 34 | 2.52 | Good | Not recommended | Machinable precision insulator for vacuum and optics |
| Steatite | 110 | 60 | 2.8 | Not recommended | Not recommended | Low-cost pressed RF and thermal insulators |
| Cordierite | 80 | 25 | 2.3 | Poor | Not recommended | Thermal-shock kiln furniture and substrates |
| Mullite | — | 80 | 2.9 | Not recommended | — | Furnace tubes and setters to 1,700 °C |
| Grade A Lava | — | 25 | 2.25 | Poor | — | Machined soft, fired hard: one-off insulators |
| Electrical Porcelain | — | 30 | 2.35 | Not recommended | — | Glazed line insulators, bushings and holders |
| Al2TiO5 | — | 15 | 3.2 | Poor | — | Best thermal-shock oxide ceramic |
Choose silicate ceramics for
Electrical insulators in volume (terminal blocks, resistor cores, thermostat bodies, spark-plug shells) in steatite or porcelain; kiln furniture, burner tiles, heater plates and honeycomb substrates that heat and cool every cycle in cordierite or mullite; vacuum, optical and RF fixtures machined to size in Macor; and casting refractories and nozzles in zircon or mullite.
Look elsewhere when
The part needs strength or wear resistance (Alumina 96 is three times stronger and far harder), toughness or a knife edge (Zirconia Y-TZP), service above 1,000 °C under load (Alumina 99.5, Silicon Carbide or Mullite for lighter loads), a heat-conducting insulator (Aluminum Nitride), or a machinable material for molten-metal or plasma contact (Boron Nitride does not wet and runs hotter than Macor).
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Silicate ceramic by application
- Machined vacuum and RF fixtures
- Macor for feedthroughs, spacers, optical mounts and coil formers — zero porosity, dielectric strength about 40 kV/mm, holds ±0.013 mm and tapped threads; Grade A Lava as the lower-cost alternative when a post-machining firing is acceptable.
- Pressed insulators in volume
- Steatite for thermostat bodies, terminal blocks, resistor cores and lamp bases; Electrical Porcelain and Technical Porcelain (C110) for glazed bushings and line hardware; Wollastonite bodies for low-loss RF parts.
- Thermal-shock kiln and burner parts
- Cordierite plates, saggars, honeycomb substrates and heater-element supports; Mullite where the load is higher or the temperature passes 1,300 °C; Al2TiO5 (aluminum titanate) for near-zero-expansion nozzles and riser tubes.
- Casting and hot-metal refractories
- Zircon (ZrSiO4) nozzles and liners in contact with molten steel and aluminum; Fused Silica crucibles and cores where purity and thermal shock matter; Silica Aerogel as the insulation behind them.
Working with silicate ceramic
- Forming & sintering
- Steatite, cordierite, mullite and porcelain are dry-pressed, extruded or slip-cast and fired at 1,200–1,450 °C with 10–15 % shrinkage; as-fired tolerance is about ±1–2 % or ±0.1 mm on small parts, and porcelain is usually glazed. Macor is cast as a glass and heat-treated to grow its mica crystals before it reaches the shop, so it arrives fully dense in rod, bar and plate.
- Machining & grinding
- Macor is turned, milled, drilled and tapped with carbide at low speed and moderate feed, dry or with water — roughly the parameters for cast iron — to ±0.01 mm and Ra 0.5 µm, and polished to 0.02 µm for optical seats. Grade A Lava machines faster still, then is fired at about 1,050 °C. Fired steatite and porcelain are diamond-ground only where a mating face demands it.
- Joining & sealing
- Macor is metallized and brazed, epoxy-bonded below 150 °C, or sealed with glass frit and holds threads for direct fastening. Porcelain and steatite insulators are cemented into metal hardware with Portland or lead-antimony cement and glazed at the joint; cordierite substrates are washcoated or clamped, never bonded rigidly across the expansion gap to a metal shell.
- Tolerances & design rules
- Design pressed silicates with a draft and uniform walls above 1.5 mm; hold precision only on machined Macor. Expansion runs from 1–2 ppm/K (cordierite) to 9 ppm/K (Macor), so match the mating metal — Macor pairs well with stainless steel and Kovar, cordierite with nothing metallic. Bending strength is modest; keep every part in compression.
Other ceramic series
Silicate Ceramics and Machinable Glass-Ceramics — frequently asked
What is silicate ceramic?
Silicate ceramics are the oldest technical ceramics — magnesium, aluminum and calcium silicates fired from clay-like bodies into steatite, cordierite, mullite and porcelain — plus the machinable glass-ceramics built on them, of which Macor (55 % mica in a borosilicate glass) is the reference. They are weaker than alumina, typically 60–140 MPa in bending, but they are cheap to press in volume, excellent low-loss insulators, and several of them can be turned, milled, drilled and tapped with carbide tooling to ±0.01 mm without any firing step.
Which grade of silicate ceramic should I use?
Macor when a precision insulator or fixture is needed in ones and tens: it machines like a hard plastic, holds vacuum, works to 800 °C continuously and does not outgas — the choice for vacuum feedthroughs, optical mounts and test fixtures. Steatite when the same insulator is needed by the thousand: pressed, glazed and fired, low dielectric loss at RF, good to about 1,000 °C. Cordierite for kiln furniture, heater supports and catalyst substrates, where its expansion of 1–2 ppm/K shrugs off thermal shock that cracks porcelain. Mullite for furnace tubes and setters to 1,700 °C with good creep resistance. Grade A Lava is machined soft in the unfired state, then fired and hardened, growing about 2 % in the process; Electrical Porcelain and Technical Porcelain (C110) are the glazed line insulators, bushings and lamp holders; Zircon (ZrSiO4) resists molten metal and has low expansion.
Which grade of silicate ceramic is the strongest?
Steatite has the highest yield strength in the series at 110 MPa; Macor follows at 95 MPa. Values are for each grade's default condition.
Which grade of silicate ceramic is easiest to machine?
Macor (machinability good, 60/100), then Wollastonite (45/100).
Macor or boron nitride for a machinable insulator?
Macor when the part needs strength, a sealed surface and dimensional precision: 94 MPa flexural, zero porosity, holds vacuum to 10⁻⁹ torr, takes tapped threads and polished seats, usable to 800 °C continuous (1,000 °C peak). Boron nitride when the part sees molten metal, plasma or temperatures beyond 1,000 °C — it is softer and slightly porous, but is not wetted by melts and runs to 2,000 °C in inert gas. Both cut with carbide; Macor is the closer match to a metal in fit and finish.
Steatite or alumina for a pressed insulator?
Steatite when the job is electrical insulation with modest mechanical load: it presses and fires cheaper, has lower dielectric loss at radio frequencies, and covers thermostats, terminal blocks and resistor cores up to about 1,000 °C. Alumina 96 when the insulator is also a structural or wear part — it is about three times stronger (300 MPa versus 120), far harder, and holds up to 1,500 °C. Both metallize; alumina does it better for brazed assemblies.