The outer pot holds 2-1⁄2 fluid ounces of water to keep the contents of the inner pot from scorching when it’s on a heat source (the warmer). But scald the pot first with boiling water (like you would a teapot), then fill it with hot water and it will keep hot hide glue in the inner pot warm enough to use for a time after you remove it from the heat source (the amount of time will vary based on ambient temperature).
The removable inner pot holds about 1 fluid ounce of glue – which is the right amount for a face frame glue-up, or for a small case project such as spice box or jewelry box.
Fill up the inner pot about one-third with hot hide glue pearls and soak them overnight, then fill the outer pot with water (hot water will make it warm faster), put the double boiler on the warmer, and the glue will be ready to use in about 45 minutes.
But this little pot is also handy when using Titebond’s liquid hide glue in colder temperatures (such as my shop at home, which is at best 50° in the winter) or Patrick Edwards’ Old Brown Glue (which must be warmed before use). Again, with water in the outer pot, squeeze some liquid hide from the bottle into the inner pot, and the glue will quickly heat to a usable viscosity and stay there – unlike with my usual technique of plunking the bottle in a bowl of hot water, which quickly cools. PWM
— Megan Fitzpatrick