Hi Cass,
The lines you’re seeing in the glaze on your cup are due to something called glaze crazing, which is completely normal for this type of glaze. There’s no need to worry as the crazing isn’t the same as cracking and it won’t affect the function, strength, or hygiene of your cup. It’s also perfectly normal for the crazing to develop a little more over time. Glaze crazing happens when the glaze and the clay body shrink at slightly different rates during firing, creating that lovely crackled or eggshell-like effect. The amount of crazing can vary depending on how thick the glaze was applied, so you might notice that not all cups have the same amount of crazing — some might not have any at all!
As per the discussions over email with you, we are working to find a solution which best suits you.

Thanks for taking the time to review our products. We hope they continue to bring you joy either at home or on the road!