In today’s lingo if you say something is “sick” it means you really like it. In glass circles, not so much, because in glass circles sick glass is a very bad thing indeed.
Sick glass is glass that is permanently etched and is foggy as a result. Sick glass is a condition in which the surface of the glass has been chemically and permanently altered. This can be caused in several ways; by allowing water or other liquids to sit in them for too long a period, from being stored in damp spaces or from washing it in a dishwasher, especially with corrosive lemon/citrus dishwasher detergent.
Sick glass is different from glass with hard water stains but often it is impossible to tell the difference between them without additional testing. In medical terms you would say that hard water damage can be treated and possibly cured but sick glass requires surgery which may or may not be successful.
Consider the glass set at the top of the page. I bought this platter and plate set at an estate sale. They came from a darkish dusty basement and while I noticed the cloudy plates I assumed because the platter was clear, that the plates were just dusty. But once I got home they appeared to be “sick.” But, while clearly foggy, I had no way to know if this was from hard water staining or permanently etched glass.
The only way to find out was the “treat” the glass. If it is just hard water deposits then there are a couple of things you can try to remove it. The first, and to me, the gentlest, is a vinegar and water soak. Make a 50/50 solution of vinegar and water and immerse your glass in it (or in the case of a vase, fill it with the solution) and let it sit for 6 – 12 hours then wash and dry. If it is hard water staining the fogginess will be gone or greatly reduced. You may need to repeat the procedure.
If that doesn’t work or only works a little then you can proceed to a similar mixture of CLR and water and follow the same procedure. Some people also swear by denture tablets, 409 and other cleaners but I think if the two methods above don’t work then you can conclude that you have sick glass and stop trying the treatments.
So what do you do for sick glass?
If your purpose for the glass is display only then there is a cosmetic repair you can make using headlight cleaner on it. This will coat the glass and make it refractive again but this is only a temporary fix and it will wear off. It’s a good repair for vases but not very practical for something like my plates.
To permanently eliminate the fogginess you will have to have the glass re-polished which gives you a new glass surface and is a permanent fix. If you have a valuable or sentimental piece of glass you will find the cost to be worth the result.