Let’s look at what is a weathering certificate and when this is required.
Weathering is required for goods to reduce the moisture content, if any.
Example UN 1408 FERROSILICON with 30% or more but less than 90% silicon, Class 4.3.
Ferrosilicon when moist will generate hydrogen which is highly flammable.
The entry in DG list UN 1403 is assigned with special provision 932 which states “Requires a certificate from the maker or shipper, stating that the shipment was stored under cover, but in the open air, in the size in which it was packaged, for not less than 3 days prior to shipment.“
For IRON OXIDE, SPENT – UN 1376 Class 4.2: IMDG Code recommends through observation column in DG List that weathering of at least 8 weeks before shipment is needed if not packed in metal drums.
So which substance require a weathering certificate?
A substance require a weathering certificate when same is asked for in the individual entry in Dangerous Goods List.