Stuck Fermentation

First, ingredients:

1.44K Chestnut Honey
2.3L spring water
1L Pressed apple juice
1 Tsp Yeast Nutrient
3mg Thiamine (Vitamin B1)
2 Tablespoons Malted Yeast Extract
3 tea bags
200g Black Currants
Muntons/Gervin G4 (Alcohol tolerance 21%)
Starting SG 1.09


The above recipe initially made for a quick and smooth fermentation, SG dropped about 80 points in less than two weeks. I racked to the secondary and decided I wanted something stronger than the 11/12% that I think this worked out at (the mead calculator was working at that time). I decided to add 0.5 kilos of honey to the secondary (heated with a little spring water and left to cool), expecting fermentation to flare up. It didn't. I left it for a few days and took another SG reading which came out at 1.04. There seemed to be fermentation because there was around 1 bubble every 45 seconds, but not what I'd expect from half a kilo of honey.

Yesterday I decided to re-pitch with K1-V116 because its an all rounder and quite tolerant. It's also much cheaper than the G4. I re-hydrated in some honey water, left for 30 mins until it was fizzing and foaming away and added to the stuck ferment. It's now been 24 hours and the must is as (in)active as it was prior to the re-pitch. I even added another tsp of nutrient and another 2mg of B1 but this did not help. Sorry for the long post - any ideas?

Thanks for your help, fellas.

Further to this, I just removed a small amount of must from the ferment to mix with the potassium carbonate, just to test what it would do; the must/potassium carbonate mix turned very dark and gave off a strong smell of peroxide. Is this normal, something to do with hydrogen being released from the must?

I definitely ordered potassium carbonate from Fast Chem via eBay. I've since read that K2CO3 makes wine musts darken a bit but subsequent fermentation will take care of it. In regards to the smell, having thought about it it was less a smell and more a kind of 'harsh' feeling in my nose... CO2 perhaps?

The ambient temperature is about 20c, must is a little cooler at 18c. The smell eventually dissipated so I added it to the must and it fizzed up, but didn't do anything noticeably untoward. Fermentation is definitely occurring but hasn't dropped a point in the last 48 hours (still at 1.04) so I'm repitching as per bernardsmith's post. I will report back.

On a side note, I'm now using GV10 (Gervin) which is meant to be good for restarting. This stuff goes off like a volcano, even in an open container!

So, repitching has worked (after two attempts) and the SG is now dropping (1 point in the past 24 hours). However, I've noticed that the must has developed a very bitter taste. I'd describe it as a 'quick' bitterness in that it doesn't last long and it then has a residual saltiness to it. Is this the taste of potassium carbonate? Is the bitterness likely to precipitate out?