Are the rings for carrying the incense altar pure gold?

If their gold casting techniques had not changed from the time they left Mt. Sinai, then the gold was not pure.

Ex 32:4 And he received [them] at their hand, and fashioned it with a graving tool, after he had made it a molten calf: and they said, These [be] thy gods, O Israel, which brought thee up out of the land of Egypt.

Ex 32:20 And he took the calf which they had made, and burnt [it] in the fire, and ground [it] to powder, and strawed [it] upon the water, and made the children of Israel drink [of it].

Pure gold does not grind to a powder, but a gold-silica mix does. The silica makes the gold hard, and even brittle, while the gold makes the silica soluble which can be used to alloy pure gold and harden it.

It is plausible that they drank a water-gold-silca mix since they are all effectively inert ingredients.

Though we don't know the exact composition of the gold used in casting in the wilderness, the material described in Ex 32 was certainly not pure gold, and it is therefore likely that the cast gold rings were of a similar material. Since there is no record of the rings stretching or breaking, and the only case of the ark falling was off the back of a cart, the rings were sufficiently hard to not stretch and sufficiently tough to not have brittle fractures under the load.

Side Note: If I remember correctly, Gold alloys in the Americas included copper to stiffen it. The least bit of moisture, and galvanic action destroyed it.