C 799, C 610 and C 530 are material classes from IEC 60672-3 (DIN EN 60672-3 in Germany). C 799 is alumina with more than 99% Al₂O₃. C 610 is a mullite ceramic with 50–65% Al₂O₃. C 530 is a fine-pored ceramic that isn't gas-tight. Older German drawings use the DIN 40685 names K 710 (also written KER 710), K 610 (KER 610) and K 530.
C 799 doesn't mean 99.7%
The class sets a floor: more than 99% Al₂O₃, plus minimum properties such as a density of at least 3.7 g/cm³. The 99.7% figure comes from makers' own C 799 materials. Morgan sells its Alsint 99.7 as type C 799, and JUMO describes KER 710 as more than 99.7% Al₂O₃. A tube sold as 99.5% alumina can sit inside the C 799 band too, if it meets the other minimums.
The classes side by side
| Class | Type | Al₂O₃ (%) | Density min. (g/cm³) | Bending strength (MPa) | Thermal conductivity (W/m·K) | Max. use temp. (°C) |
|---|---|---|---|---|---|---|
| C 610 | Mullite | 50–65 | 2.6 | 120* | 2–6 | 1200 |
| C 620 | Mullite | 65–80 | 2.8 | 150* | 6–15 | 1200 |
| C 780 | Alumina | 80–86 | 3.2 | 200 | 10–16 | 1200–1400 |
| C 786 | Alumina | 86–95 | 3.4 | 250 | 14–24 | 1400 |
| C 795 | Alumina | 95–99 | 3.5 | 280 | 16–28 | 1400–1500 |
| C 799 | Alumina | >99 | 3.7 | 300 | 19–30 | 1400–1700 |
C 530 isn't in that table. WIKA describes it as not gas-tight, fine-pored, highly resistant to changes in temperature, usable up to 1,600 °C. It is used as an outer tube around a gas-tight inner tube.
Which grade for which job
These rules come from the data sheets of two German thermocouple makers, WIKA and JUMO, and from Rössel's handbook.
- Platinum thermocouples (types S, R, B) above 1,200 °C. WIKA says to use gas-tight ceramic, preferably high-purity C 799, because the wires can be poisoned. JUMO says that for platinum thermocouples both the insulating rod and the protection tube should be KER 710.
- Reducing atmospheres. Keep C 610 out. JUMO gives KER 610 as 60% Al₂O₃ and 37% SiO₂, and notes that in a reducing atmosphere as little as 0.2% SiO₂ in the tube or insulator is enough to form brittle platinum silicides.
- Thermal shock. WIKA rates C 530 highly resistant to changes in temperature and C 799 only partly resistant. C 799 assemblies should be preheated and pushed into the furnace slowly. When you need both, WIKA pairs a porous C 530 outer tube with a gas-tight inner tube.
- Base-metal thermocouples (K, N, J). Insulators are commonly C 610, with 1 or 3 mm wire (WIKA). Rössel lists 1.0, 1.38, 2.0 and 3.0 mm wire for base metals and 0.5 or 0.35 mm for platinum types, with C 799 insulators for the latter.
- Alkali vapours. WIKA lists both C 610 and C 799 as not resistant.
- Cost. JUMO puts KER 610 at about a fifth of the price of KER 710.
Maximum temperatures: the sources disagree
The Brevier table gives 1400–1700 °C for C 799. WIKA rates C 799 tubes to 1,600 °C. For C 610 it is 1,200 °C in the Brevier and 1,500 °C at WIKA. Each number is someone's figure for their own material or design, so ask for the limit of the specific part. Our brochure gives 1600–1800 °C for corundum and 1300–1450 °C for high-alumina, and we confirm the figure per part in the quotation. Not every corundum part is rated to 1800 °C.
How our grades compare
| Our grade | Al₂O₃ (%) | Where the Al₂O₃ content falls |
|---|---|---|
| Corundum | 99.7 | Above the C 799 floor (>99%) |
| Corundum | 99.5 | Above the C 799 floor (>99%) |
| Corundum | 99 | On the line between C 795 (95–99%) and C 799 |
| High-alumina | >85 | Above the mullite classes C 610 and C 620 (up to 80%) |
One alumina tube sample, tested by Chinalco Testing Technology (Zhengzhou) Co., Ltd. in May 2026 (report 26-04928), came out at 99.61% Al₂O₃ and 0.15% SiO₂. That is one sample, not a figure for every lot.
We don't claim C 799 or C 610 for our tubes. A class needs the minimum properties as well as the Al₂O₃ content, and we haven't had our grades tested against the full list. Our brochure density for pressed and extruded corundum is 3.65–3.8 g/cm³, which straddles the C 799 minimum of 3.7. If your drawing names a class, send it: we quote a grade and send the test data for it. Material data for both grades is on the materials page.
KER 710, KER 610 and other names you'll see
| Class now | Old name (DIN 40685 / VDE 0335) |
|---|---|
| C 799 | K 710, also written KER 710 |
| C 610 | K 610, also written KER 610 |
| C 530 | K 530 |
For sizes that go with each grade, see thermocouple protection tube sizes.
Sources
- Brevier Technische Keramik (Verband der Keramischen Industrie), Table 41: properties by DIN EN 60672 class
- WIKA data sheet TE 65.80, thermocouples with ceramic protection tube
- JUMO data sheet 90.1000 (German), KER 710 and KER 610
- Rössel-Messtechnik, Praxis der Thermoelemente (German), Table 9 and insulating rods
- DIN Media: DIN 40685-1, withdrawn
- Our own figures: the factory's standard-size list, 2026 orders and test report, as recorded in our product catalog.