At the garage sale where I scored all my sweet Pyrex Butterprint, the seller threw in three of these Ball jars and lids/seals for free.
I was thinking they would be perfect for crockpot dulce de leche!
Did you know that you can identify the age of your Ball jars by looking closely at the logo font? These here are not very old - if the "B" has a loop in it, the piece is From before 1960. You can see an example at my post on Ball mason jars.
Back to the dulce de leche, just pour some sweetened condensed milk into your mason jars and screw on the lids. Pop them into your slow cooker and cover with water, then cook on low for 8 hours and you will have a creamy, caramelly treat waiting for you!