At 600C the only adhesives are ceramic ones. What are you bonding, what is the purpose of the bond, and what is the joint geometry? Thermal expansion stresses will be one of the main issues.
I would agree with Kirby Wan. There are quite a number of glass and glass ceramic seals that would stand the temperature, but I would be very cautious about the longer term resistance to the thermal cycling. You might also think about a graded seal to lower the thermal stresses at the crtical interface.
Here are three manufacturers, Suereisen, Aremco, and Cotronics, that make high temperature adhesives. I have used products from all three with very good results. We preferred Sauereisen products. This was not because they were better, but what we started with.