Two interesting responses on heat checks in glass. Long time glass dealer, Randy Bly of Birmingham, Alabama gives his opinions on the effect of heat checks.
Thank you for the interesting subject. I appreciate the different opinions. From a dealers perspective I consider heat check to be outright damage. Glass has unique properties. In perfect form and condition it creates a positive emotional reaction in me. A damaged piece seems to only bring me disappointment and frankly just bums me out. It grossly affects it’s value. I feel it will reduce a $200.00 piece to $15.00 and a $10,000.00 piece to$300.00. Every time I look at it all I can think is ” I wish it wasn’t broken”. And then just try to sell it to anybody! I made the rule for myself 30 years ago don’t buy damaged pieces. Nothing but heartache! Damaged glass reminds me of only one thing……..all the Kings horses and all the Kings men…………..
Thanks for letting me express my opinion

Next, Wayne Montano of Montano’s Antique Glass Repair of Emmett, Idaho provides this interesting insights based on his line of work experience.
There sure are a lot of different opinions on heat checks. I’m not a glass blower and never worked in a factory where items were made and a finished product was sold. But I would think that most factories would recycle into the cullet pile those pieces that did not anneal properly
and developed after the piece was ready to ship. What I have learned over the years in doing glass repair (since ’81), is that a piece of
glass (handle) will crack or separate at the attachment point of lets say a cruet. This I believe is from the making and stress built up in
the piece and just lets go. I’ve had a piece do this in one of our cabinets. It was fine when I put it and it was cracked when I took it
out. It is my opinion that a heat check is just as serious as a crack in any glass. Whether it is done from the stress in the making of the piece
or something bumps it. A crack is a crack is a crack.

When we buy and sell glass, we sell items that are cracked at 10% of the mint value. If a piece were chipped and the repair can not be detected, we sell an item at 90% of its mint value. Or if a piece can not look mint, the value we put on it varies around 50% of the mint value. But back to a crack. Once you have one we can hide about 90% of the
refraction of the break. But at this time, I don’t know of anyone doing a 100% invisible repair on a crack. There are many that claim it, but to this date I personally have seen none. On a crack, what your seeing isthe air between the two layers of glass. Once this air is removed, then 90% of the refraction disappears.

Speaking of stress in glass. We were repairing about 8,000 chipped goblets a year for about a 20 year period. On occasion, after removing a
small amount of glass from the top rim, the glass would release its built in stress. I’ve had the glass crack about 1/4 inch from the rim,
then in a clean cut around the rim, the top would separate from the glass. I’ve had several do this while working on the piece and had them
do this after the work was done and washed and dried ready to go back to the customer. There is a tool that is called a Polaris scope that can be used to show the stress built into glass. I’ve never had one, but I have seen them used in a lamp working shop.

Hope this helps the discussion. Always like sharing info on what little I know.