Smelting Iron Ores

As I was mining coal, I realised I had 771 iron ores in my bank. So, I ran to the bank, stored my Addy pickaxe, 12 coals and an uncut sapphire. I after I've cut the sapphire, I took a full batch of iron ores and went to the furnace and smelted them. Here are the results:

Batch 1 = 16 iron bars
Batch 2 = 11
Batch 3 = 15
Batch 4 = 10
Batch 5 = 13
Batch 6 = 15

Total = 80 iron bars /168 attempted