I just tried to clean out a 6.5 gallon carboy that I had used for primary fermentation. First, I hit it with hot water and dumped it out. Then, I put about a half-gallon of hot water with some dish soap. No more than five seconds after I began scrubbing the carboy with an angled brush, there was a loud explosion, and the carboy broke into four pieces in the sink. The temperature of the beer was around 60°F, and the temperature of the water was about 125°F. What, if anything, did I do wrong? Was it just a bad carboy or did I commit a fundamental mistake? What is the proper way to clean a glass carboy?

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    I'm pleased to hear you weren't injured. But why do homebrewers use glass? Although there's talk of oxygen permeating plastic, the affects of that are small, and definitely a non-issue if the rest of your fermentation process isn't oxygen free (anything but racking via CO2 to a purged keg.) Using glass should be the last thing on a homebrewer's list, not the first. By the time you've got to the end of that list (after temp control, O2 cylinder, etc..), you'll probably hop over glass carboys and invest in a stainless conical!
    – mdma
    Commented Nov 29, 2013 at 1:21
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    This is my first time doing the primary in glass. My reasons for using glass (and I'm still a novice brewer, so if what I say is ridiculous, don't be surprised) is, first, that I've been told that scratches in food-grade plastic cannot be sanitized, and I've noticed scratches in the bottom of my bin, so I switched to glass. The other reason is because, since I've always done secondary in glass, I enjoy being able to glance at the carboy and see the status fermentation without having to pop the lid of the bin. Too often, I just take time out of my day to do nothing but stare at the beer. Commented Nov 29, 2013 at 5:12
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    First off, they do make plastic carboys that you can gaze in wonder into. Google "Better Bottle". The only reason I would use a glass carboy for is for long term aging (many, many months, or years), for the reasons mdma specified. I remember listening to a Brewing Network podcast where Jamil talked about having a sour's pellicle fall back into the beer after 6 months due to oxidation where it would last 12+ months without the introduction of oxygen. At that point though, I'd probably move on to kegs where I can easily sample it, avoid skunking, and store with less horizontal space.
    – Scott
    Commented Nov 29, 2013 at 5:15
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    Hi Daniel, what you say makes sense and is the reasoning a lot of homebrewers start with glass, however there are good plastic carboy alternatives, and with the right cleaners (PBW,Oxiclean) you never need to scrub, so no scratches. They're lighter and much easier to handle (I drop mine empty all the time - they just bounce) and they don't risk serious injury.
    – mdma
    Commented Nov 29, 2013 at 11:16
  • While you're contemplating replacing your lost carboy, looks like you're in luck, being Black Friday and all: austinhomebrew.com/…
    – Scott
    Commented Nov 29, 2013 at 15:08

5 Answers 5


Don't use hot water to clean glass carboys. The glass is subject to thermal shock when some of it is heated while the rest remains cool. Count yourself lucky that you escaped without injury.

  • I'd be amazed that as low a temperature as 125F would induce thermal shock, though that's obviously what must have happened. Is it because the glass is particularly thick? How big a temperature differential will a carboy stand? Commented Jul 23, 2014 at 11:58

I think that any rapid change in temperature can explode glass.

The same can happen to a drinking glass that you pull out of a hot dishwasher then add cold beer. It can crack and possibly explode.

I am not saying that all glass does this but in some cases it can.


I have to say, I am surprised by this. I'm glad you brought it up.

I think the temperature gradient could have been the problem. The bottom with the water was rather hot and the top was cool causing stress over the long length of the bottle.

On the other hand, I'm surprised that the gradient caused by hot tap water and cool but not cold air temperature would have been enough of a shock. I wonder if there was already a crack or something.

On the other other hand it's been my observation that many glass carboys these days are somewhat thin in relation to their size these days and so are not as robust. I compare this to glass cooking bowls that are thicker and smaller than a carboy and so are able to withstand boiling water being dumped in them. Also, the glass in cooking bowls is possibly of higher quality.


Something to watch out for as I bottle my weisbier and clean my 6.5 gal. glass carboy. :-(

Here's a possibility: I remember in physics labs some students would wrap especially large glass flasks and glass thermal bottles in fiber reinforced tape so that if there is breakage at least the mess and the danger was somewhat contained. In our case it might spoil the look of our beer in the carboy but it might be worth it. Or maybe there is a plastic web sleeve or sock we could get (google carboy shield).

  • The gradient isn't top to bottom, but inside/outside - immersing the carboy in hot water warms the outside, which expands more than the inside, causing the glass to crack.
    – mdma
    Commented Dec 13, 2013 at 15:17

I bought a glass carboy for the same reasons as you. however I quickly discovered I can buy about 6 buckets for the price of 1 carboy and they are WAY easier to clean. My glass carboy is now filled with about 4 strands of blinking colored Christmas lights and sits on top of my fridge on the patio. As soon as my Keezer is done (hopefully this weekend) it will have a new home :)


Never clean glass or plastic carboys with hot water and soap. Use a proper cleaning agent like Straight A or PBW (Powdered Brewery Wash). I have a no scrub method where I use PBW in my carboys and soak them overnight or longer. You can use PBW repeatedly without it losing effectiveness so generally I leave it in the carboy until I'm ready to use it again and then transfer it into the next carboy that needs cleaning. All the hop material and yeast that form a ring around the top of the carboy falls out and the carboy is ready for use after properly sanitizing it. No scrubbing or carboy brush is necessary.

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