Did a Russian Imperial Stout a week ago. Fermented well. I had the fridge set at around 63. I noticed that the stick on temperature gauge on the side of my fermenter read 77! Could it really have been 10+degrees warmer in the fermentation chamber than the ambient temperature? If so, will the beer taste astringent? I checked a sample for FG today, and it didn’t’ taste notably astringent. Total abv=9.6%
It shouldn’t affect astringency, but it could increase the ester production and possibly fusel alcohol production.
Fermentation temp doesn’t have anything to do with astringency AFAIK.
Thought astringency was caused by fusel alcohols. If not, what do fusel alcohols taste like?
They taste like hangovers. 
But seriously, they just taste harsh and solventy. I’ve heard jet fuel, nail polish remover, paint thinner, permanent marker, etc. But I really just get rough and harsh.
I guess another question I have is, is it common to get 10+ degree difference between the fermentation vessel and the outside temp during a vigorous fermentation?
No, astringency is often caused by not maintaining proper pH during the mash and sparge phase. Fusel alcohols are formed during an excessively warm fermentation, and have a harsh, hot, solventy flavor (and can give you a hell of a headache the next day). A high OG beer like a RIS has tons of fermentable sugar, so fermentation temps can spike several degrees higher than ambient during high krausen.
10+ degrees higher? Is it possible that my sticky temperature gauge is wrong?
They’re supposed to be accurate to within a couple degrees F. Did you cool the wort to 63F as well?
Yeah, cooled down to about 63. Does anyone use that metal hollow probe that you can put directly into the fermentation vessel for direct temperature readings?
Do you think my beer will taste bad due to fermentation at that higher temp?
It’s possible, but don’t give up too early - RIS needs a long time to mellow. You may hate it at 2 or 3 months, but at 6 or 8 months the beer may come into its own. Give it plenty of time before you pronounce it a disappointment. Good luck !
Rocket fuel. You’ll likely want to age this beer for a good year or more.
Does age actually reduce fusel alcohols?
You have a few things on your side. First off you pitched cool and the first part of fermentatation is where most of your fusels are generated. Second, if you used a clean ale yeast like WY1056/WLP001 or US-05 these yeasts are actually fairly clean into the mid to high 70s. I had an Imperial amber that got up to 77 degrees during high krausen using WY1056 and it was still pretty clean, though I did get a little bit of a head ache after a couple pints.
They are supposed to. It certainly does not hurt to put the beer away for awhile and be patient.
Nail polish remover
Given time, fusels will usually age out into esters.
Now I wonder what nail polish remover tastes like… Perhaps with a splash of OJ and some mineral spirits.