Click on a furnace to open the Smelting menu, scroll down on the left side menu until you get to Casting Metals and then click on the gold bar (or any or the crafting metals) to bring up the crafting options. Molten Glass is at the bottom of the center column options.
This is a very good description on how to make molted glass. I went to another post about it and it did say say you click casting, but nothing about actually clicking the bar slots. I was just looking under casting without clicking the bars and could not find it, but after seeing that you mentioned crafting I thought to click the CASTING metals gold, silver, steel, or iron BAR slots, even if you do not have a bar in it, which takes you to the jewelry making section and from that scroll past all the jewelry all the way down to the bottom and there it is. I did have to go into setting though and do the destroy buckets when farming. I appreciate this so much. I will copy your code and post it on the other post.
Tren, sorry , after rereading your post I saw that you did say click on the bars, I just thought if you didn't have any bars it wouldn't work , so I clicked like you said and there it was all the bottom. My brain just did not connect the crafting bars and how they connected to making glass.