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Note 2 - Inventory (Tables)
 3 Months Ended
Mar. 31, 2020
Notes Tables  
Schedule of Inventory, Current [Table Text Block]
   
20
20
    
201
9
  
Furnace parts and supplies
   $
925,300
     $
621,991
  
Raw materials
    
1,967,234
      
2,125,921
  
Work in process
    
1,877,131
      
1,624,499
  
Finished goods and filtration systems
    
1,209,729
      
1,492,135
  
Reserve for obsolescence
    
(654,895
)
    
(665,308
)
Net Inventory
   $
5,324,499
     $
5,199,238