Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
OPERATING ACTIVITIES:
Net income	$ 19,685	$ 27,148	$ 17,754
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	17,603	16,549	16,141
Amortization of deferred financing costs	14	15	14
Deferred income taxes	20	(7,612)	648
Loss on disposition of property, plant and equipment	624	377	622
Stock-based compensation	1,489	1,405	1,036
Changes in operating assets and liabilities:
Trade receivables, net	(3,129)	308	(892)
Inventories, net	700	(1,113)	2,295
Prepaid expenses and other current assets	1,377	(1,594)	308
Other assets	(1)	5	4
Accounts payable and accrued expenses	438	(1,130)	532
Other liabilities	(85)	(76)	(615)
Net cash provided by operating activities	38,735	34,282	37,847
INVESTING ACTIVITIES:
Purchase of property, plant and equipment	(53,762)	(21,337)	(17,664)
Acquisition of assets of a business			(50)
Proceeds from sale of property, plant and equipment	605	592	208
Net cash used in investing activities	(53,157)	(20,745)	(17,506)
FINANCING ACTIVITIES:
Cash dividends paid	(3,022)	(3,013)	(2,782)
Proceeds from exercise of stock options	73	73	155
Purchase of treasury shares	(411)	(309)	(2,928)
Net cash used in financing activities	(3,360)	(3,249)	(5,555)
Net (decrease) increase in cash and cash equivalents	(17,782)	10,288	14,786
Cash and cash equivalents at beginning of period	85,000	74,712	59,926
Cash and cash equivalents at end of period	$ 67,218	$ 85,000	$ 74,712