Quarterly report pursuant to Section 13 or 15(d)

Condensed Consolidated Balance Sheets (Unaudited) - (Parenthetical)

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Condensed Consolidated Balance Sheets (Unaudited) - (Parenthetical) - USD ($)
Jun. 30, 2018
Dec. 31, 2017
Assets    
Accounts receivable, allowance for doubtful accounts $ 5,000 $ 35,000
Property, plant and equipment, accumulated depreciation 53,278,938 50,451,436
Intangible assets, accumulated amortization $ 11,710,526 $ 10,568,479
Shareholders' equity    
Common stock, par value (in dollars per share) $ 1 $ 1
Common stock, shares authorized (in shares) 24,000,000 24,000,000
Common stock, shares issued (in shares) 10,300,000 10,300,000
Common stock in treasury, at cost (in shares) 1,497,793 1,566,769