I placed 900 sand and 900 silty soil in 6 different furnaces.
Each one produced a different amount of glass blocks!
It ranged from around 350 to 450.
I ended up with 2403, averaging at just over 400 glass blocks per Furnace.
The recipe is for 2 sand + 2 silty soil so I should have ended up with 2700 blocks (450*6).
And I am 100% sure that I put 900 of each ingredient in every furnace, I went out of my way to ensure this so I was making as much as possible.
and you also checked you had enough fuel in all of them? just ticking off the most obvious reasons, I assume you did that
Yeah, loads of fuel!
There was no ingredients left in the furnace, so it used them all up, there was excess fuel remaining.
Isn’t there a skill/guild perk that gives you extra crafted goods randomly. So if you make 100 brews you could actually make 112.
Pretty sure it’s a guild perk?
Yes, there is.
However, I don’t have it, plus that would give me extra items, I have less than expected!
Your both raw material slots are empty when it is done? Could be color issue with smart stacks.
Oh I see, I read that wrong - just woke up 
No worries 
Yep, both empty, and I’m pretty sure it was all the same colour too!
Bonus consumables, guild buff, only works on ehh, consumables like food and brews, nothing else, SADLY! 
I knew it! Thought I might have dreamt up some Boundless updates again… 
Again? Pray tell what awesome ideas you’ve had while dreaming!!
Oh I’d have to compile my list and release it as a hardback 
Has this happened once for those 6 furnaces, or have you been able to replicate this?
I’ve not been able to reproduce this so far. Do you have different types of Furnaces? Also, what fuel types are you using?
I tried again, but it worked fine with 2 furnaces. Though these had the same fuel (enriched hard coal).
The original 6 titanium furnaces 2 had enriched hard coal and 4 had enriched combustion particle.
I’ll try with the ecp now, as I seen to remember emptying these first, and that was where there was less had than expected.
Not sure if it would make a difference, I had logged off over night, then returned to the furnaces later.
Only use titanium furnaces.