Ginger did extensive research (as she always does) on bonding clay together.  You will find her in-depth article here: Bonding Polymer Clay to Itself - Testing Bakeable Bonds - The Blue Bottle Tree

Use any liquid clay to enhance the bond of raw to raw polymer clay, especially when using Premo, Souffle, or Kato. When bonding baked Kato to itself, use Loctite Control Gel.

Here's another article: What's the Best Glue for Polymer Clay? - The Blue Bottle Tree

What’s the Best Glue for Polymer Clay?

• To bond uncured polymer clay to another piece, it’s best to use liquid clay or other bakeable glue such as Kato Poly Paste or Sculpey Bake and Bond.
• To bond two pieces of cured polymer clay that will not be baked again, use a flexible superglue such as Loctite Gel Control, Zap-a-Gap, or Gorilla Glue GEL Superglue.
• 2-part epoxy is another good glue for use with polymer clay. It can become brittle with age and pop off.
• To bond metal to polymer clay, it’s best to bake the metal into the clay. Glue will not be reliable for this bond over time.
• E6000 is highly recommended by others, but it often does not properly set on polymer clay or becomes brittle with age. I think there are better alternatives.
• To bond polymer clay to wood, paper, or fabric, use a strong white or PVA glue such as Weldbond.
• There is no perfect glue for all uses. Polymer clay is plastic and can be flexible, making a durable bond challenging.