Custom High-Temperature Alumina Ceramic Tubes for Furnaces

Frequently Asked Questions About Alumina Tubes

Should I choose 99.5% or 99.8% purity for my application?

For most furnace work below 1500°C, the 99.5% grade is the practical choice: same gas-tightness, same chemical resistance, lower cost. The case for stepping up to 99.8% is specific: higher mechanical strength (70,000 PSI flexural vs. 50,000), higher dielectric performance (300 V/mil vs. 270), and a higher working temperature ceiling (1800°C vs. 1750°C). If you’re insulating high-voltage heating elements, running near the upper temperature limit, or need every bit of structural margin you can get, 99.8% pays for itself. Below that, 99.5% does the job.

What ramp rate should I use to avoid thermal shock?

Alumina is tougher than most ceramics, but it’s not quartz, and you can’t quench it. As a general guideline, ramp at 5°C per minute or slower for both heating and cooling, especially through the first cycle of a new tube and across the 200–600°C range where thermal stress builds fastest. For larger-diameter tubes, slower is better. The exact safe ramp depends on tube geometry, wall thickness, and the temperature range you’re traversing. If you’re pushing the limits, send us your duty cycle, and we’ll talk through it.

Can these tubes reach 1800°C without sagging?

Reaching 1800°C, yes, the 99.8% grade is rated to that maximum working temperature. Holding shape there is a separate question. Our published sag temperature is 1600°C, defined as the point at which an unsupported 1-1/4″ tube spanning 18″ will deflect 1/4″ in 1 hour. Above that, the tube needs to be supported along its length to remain straight under its own weight or any sample load. For continuous operation near 1800°C, support fixturing isn’t optional.

Are these tubes gas-tight for vacuum or controlled-atmosphere use?

Yes. Both the 99.5% and 99.8% grades pass the ASTM dye-penetration test for gas-tightness, which is the relevant standard for vacuum and inert-atmosphere work. Fully dense alumina has effectively zero open porosity, so oxygen won’t migrate through the tube wall and contaminate a reducing-atmosphere process. The seal between the tube and your furnace flange is the more common failure point. Make sure your O-ring spec matches the OD tolerance of the tube you’re ordering.

What configurations are available?

The standard sizing table covers open-end tubes, which are the most common configuration for process work. Contact us with your dimensions, and we’ll confirm specs and lead time.