I cooked it. Now it's the next day, and it's more gelatinous, but still fairly spreadable, so I'm thinking it's still good to use.

I don't cook my glue. I just use wallpaper paste and water. I mix it up in a ziploc bag and just seal it in between uses.

It does get a little thicker, but I can always think it down with water again if needed and shaking the bag until it is incorporated.

Thanks Sue, I'll definitely make a mental note in my head about that.

Do people just use the cornstarch instead of flour?