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Note 2 - Inventory (Tables)
 3 Months Ended
Mar. 31, 2019
Notes Tables  
Schedule of Inventory, Current [Table Text Block]
   
201
9
    
201
8
  
Furnace parts and supplies
   $
654,278
     $
596,806
  
Raw materials
    
1,605,809
      
1,659,826
  
Work in process
    
1,581,128
      
1,691,175
  
Finished goods and filtration systems
    
1,401,445
      
1,596,042
  
Reserve for obsolescence
    
(945,945
)
    
(1,111,794
)
Net Inventory
   $
4,296,715
     $
4,432,055