12. Revenue

The following is a disaggregation of revenue categorized by commodities sold:

	Three-month			Three-month			Nine-month			Nine-month
	period ended			period ended			period ended			period ended
	September 30,			September 30,			September 30,			September 30,
	2018			2017			2018			2017

Silver
Provisional sales revenue	$	4,563		$	5,813		$	16,501		$	23,986
Derivative pricing adjustments		(298	)		151			(343	)		1,466
		4,265			5,964			16,158			25,452
Zinc
Provisional sales revenue	$	8,941		$	1,406		$	32,104		$	7,655
Derivative pricing adjustments		(2,494	)		330			(2,726	)		544
		6,447			1,736			29,378			8,199
Lead
Provisional sales revenue	$	6,785		$	5,475		$	22,641		$	18,461
Derivative pricing adjustments		(489	)		213			(440	)		312
		6,296			5,688			22,201			18,773
Other by-products
Provisional sales revenue	$	133		$	300		$	360		$	1,927
Derivative pricing adjustments		(12	)		1			3			2
		121			301			363			1,929

Gross revenue	$	17,129		$	13,689		$	68,100		$	54,353
Treatment and selling costs		(5,395	)		(3,875	)		(18,632	)		(12,119	)
	$	11,734		$	9,814		$	49,468		$	42,234

Derivative pricing adjustments represent subsequent variations in revenue recognized as an embedded derivative from contracts with customers and are accounted for as financial instruments (see Note 16). Revenue from contracts with customers is recognized net of treatment and selling costs if payment of those amounts is enforced at the time of sale.