Charles Ellicott Commentary Exodus 30:3

"And thou shalt overlay it with pure gold, the top thereof, and the sides thereof round about, and the horns thereof; and thou shalt make unto it a crown of gold round about." — Exodus 30:3 (ASV)

You shall overlay it with pure gold. — Next to the Ark of the Covenant, the most holy article of furniture contained either in the sanctuary or in its court was the altar of incense. It symbolised prayer in its general use (Psalms 141:2; Luke 1:10), and it symbolised expiation in the purpose to which it was to be applied on certain occasions, as when the high priest had sinned in his official capacity (Leviticus 4:3–12), or when the whole congregation had sinned through inadvertence (Leviticus 4:13–21). It was, therefore, most holy to the Lord. Hence, its materials were to be the same as those of the ark of the covenant, and its place was to be directly opposite the ark, near to it, but on the outer side of the veil (Exodus 40:5).

A crown of gold round about. — Compare what is said of the table of shewbread (Exodus 25:24). In both cases, a raised rim or edging is meant, which would prevent what was on the top from falling off.