UN/NA Number Reference
DOT Class 4.1 Flammable Solid placard

UN 1346: Silicon powder, amorphous

49 CFR 172.101 Hazardous Materials Table·ERG 2024

DOT Classification and Shipping

UN 1346 is assigned the proper shipping name Silicon powder, amorphous in the U.S. DOT Hazardous Materials Table (49 CFR 172.101). It is classified as a Class 4.1 Flammable Solid material, Packing Group III. Required label(s): 4.1.

Packing group indicates the degree of danger within a hazard class: PG I is high danger, PG II medium, PG III low. The proper shipping name, hazard class, UN number, and packing group together form the basic shipping description that must appear on shipping papers and, where required, on the package and placard.

Emergency Response (ERG Guide 170)

Hazards: Flammable. May react with water producing flammable hydrogen gas. May reignite after extinguishing.

Fire: DO NOT use water, foam, or CO2. Use dry sand, dry sodium chloride powder, or Met-L-X.

Spill: Do not touch. Cover with dry sand or earth. Keep dry. Do not use water.

Chemical and Physical Properties

Silicon powder, amorphous appears as a dark brown powder. Insoluble in water and denser than water. Burns readily when exposed to heat or flames, and may be difficult to extinguish. Water may not be effective in extinguishing flames. Used to make computer microchips.

CAS Number: 7440-21-3
Molecular Formula: Si
Molecular Weight: 28.085 g/mol
Color / Form: Black to gray, lustrous, needle-like crystals or octahedral platelets (cubic system); amorphous form is dark brown powder
Boiling Point: 4271 F
Melting Point: 2570 °F (NIOSH, 2024)
Specific Gravity: 2.33 (sinks in water)
Vapor Pressure: 0 mmHg (approx)
Water Solubility: Insoluble

Regulatory Data

OSHA PEL
TWA 15 mg/m 3 (total) TWA 5 mg/m 3 (resp)
NIOSH REL
TWA 10 mg/m 3 (total) TWA 5 mg/m 3 (resp)
IDLH
N

Special Provisions

49 CFR 172.102 special provision codes for this entry: A1, IB8, IP3, T1, TP33. These codes modify the general requirements (packaging, quantity limits, exceptions) for this specific material. Look up each code in 49 CFR 172.102 for the full text.

Need the full interactive record? Open UN 1346 in the SpillNerd lookup tool for the live database entry with placard graphics, GHS pictograms, full segregation tables, and synonym search. For another material, use the UN number and chemical lookup.

Reference data compiled from the DOT Hazardous Materials Table (49 CFR 172.101), the 2024 Emergency Response Guidebook, and public chemical databases (PubChem, NIOSH, EPA). Always verify the correct shipping description against the current regulation and the manufacturer's SDS Section 14 before shipping.