Oh, no. I meant hatcheck. Dick Stark of Bethesda, Maryland observes.
No such thing as a heat check anyway… it’s when it cools that it checks. Right ?

Marshall Ketchum of Genoa, New York weighs in
A heat check is, usually, a small crack that occurs because the glass has not been properly annealed. Annealing glass removes strains that are built into the glass when it is worked. A heat check can occur when the glass cools or can occur later, sometimes months or years later. They often occur where two pieces of glass are joined such as handles applied to a bowl. The heat check may be internal and not come to a surface or you may be able to feel it with a finger nail. Heat checks can be stable and, once formed, not get any worse. On the other hand the heat check can progress until it forms a real crack and in the worst case the part with the check/crack can separate from the rest of the glass.

A heat check is usually quite visible. The check/crack makes a dramatic change in the index of refraction of the glass. You just need to look for them particularly where two different pieces of glass are joined.

A heat check will usually make a significant difference in the value of the piece. One of the problems is that once you buy a piece with a heat check you can never really be sure that it won’t get worse. If it does get worse it may affect the value even more. I try to stay away from pieces with a heat check unless it is a quite rare piece and it can be obtained at a bargain price.

Finally, Charles Sweigart gives his opinion on the antics of Bonnie & Clyde
Internal cracks, especially where handles or other applied glass meet the body of the object have always been referred to as “heat checks”.

I believe that improper annealing techniques cause most of those defects.

To show proof I will tell of something I observed several years ago. My brother-in-law acquired many glass marbles/spheres about 4-5” in dia. He put them in a device with 2 or 3 big rubber cups each attached to a motor which supported the sphere and rotated the cops to which polishing compound was applied. About half of the spheres would pop loudly while the process was running as large cracks opened internally. It appeared that polishing caused stress relief in improperly annealed large spheres. Some even cracked while sitting on shelf before polish.

Any defect affects the value of most any item. It simply opens the door to negotiate a lower price for the buyer. How much more would “Venus de Milo” be worth if she had her arms? On the other hand, what would Bonnie & Clyde’s car be worth without the bullet holes?