## Common ground
All options are covers for crucibles used in high-temperature laboratory or industrial processes.
## Options at a glance
**3926.90 – Plastic**
- **Defining trait:** Made from heat-resistant plastic, suitable only for low-temperature applications.
- **Typical examples:** Polypropylene or PTFE lids for chemical storage or mild heating.
- **Best fit when:** Used in non-high-heat environments (e.g., room temp to ~150°C).
**6903 – Refractory ceramic**
- **Defining trait:** Made from dense, high-melting-point ceramics designed for extreme heat and chemical resistance.
- **Typical examples:** Alumina or zirconia crucible lids for metallurgy or glass melting.
- **Best fit when:** Operating above 1000°C with thermal shock exposure.
**6909.1X – Ceramic**
- **Defining trait:** General ceramic material, may include non-refractory types; less heat-resistant than refractory ceramics.
- **Typical examples:** Porcelain or stoneware lids for moderate heating (e.g., lab drying).
- **Best fit when:** Moderate temperatures (up to ~800°C) and lower thermal stress.
**7017 – Glass**
- **Defining trait:** Made from borosilicate or similar glass; transparent, chemically inert, but fragile under rapid heating.
- **Typical examples:** Borosilicate glass lids for lab crucibles in controlled heating.
- **Best fit when:** Visual monitoring needed and temperatures below 500°C with slow thermal changes.
## Key differences
| Feature | 3926.90 (Plastic) | 6903 (Refractory ceramic) | 6909.1X (Ceramic) | 7017 (Glass) |
|--------------------|-------------------|----------------------------|------------------|--------------|
| Max temperature | Low (~150°C) | Very high (>1000°C) | Medium (~800°C) | Moderate (~500°C) |
| Thermal shock | Poor | Excellent | Fair | Poor |
| Transparency | No | No | No | Yes |
| Chemical resistance | Good (plastic) | Excellent | Good | Excellent |
## How to choose
- Does it need to withstand temperatures above 800°C? → 6903
- Is transparency required for observation? → 7017
- Is it used at moderate heat (up to 500°C) with no thermal shock? → 6909.1X
- Is the temperature below 200°C and non-corrosive? → 3926.90
## Overlaps & pitfalls
- **Plastic vs. ceramic/glass:** Plastic may be mistaken for low-temp ceramic; avoid if heat exceeds 150°C.
- **Ceramic (6909.1X) vs. refractory (6903):** Refractory ceramics are a subset of ceramics; use 6903 only for extreme conditions.
- **Glass vs. ceramic:** Glass is fragile under thermal stress; not suitable for rapid heating.
- If unsure, prioritize the product’s **main function**: high heat → 6903; visibility → 7017; moderate use → 6909.1X; low temp → 3926.90