What is the most notoriously hard borosilicate color to work with?

The more minerals the harder it is to work.  So jet black could be that.  But then there are also colors like cadmium based that too much heat will cause to boil.  Cadmiums we feel are the hardest to work as they take the most time.   No color is really harder to work than another, other than the time it takes.

Adding dichor or opal can also make the color harder to work per say.  It does not really make it harder to work either, but if you work it too hot, the dichro or opal will burn out.

 

Why do the Gordo Scientific Riptide CC carb caps have 2 air intake holes?

This is a cool feature about those, depending on which hole you carb or cover, it will change the direction of the airflow coming in. So if using terp pearls, it will change the direction that your pearls spin. These are super awesome for someone who wants to get the absolute most out of their terp pearls and have a lot of fun!  I mean that is what life is all about right!  Having fun!