Glamorgan bowled out Sussex for 327 in a topsy-turvy day at Hove.
Despite a strong start from opening duo Salt and Wells, Sussex slipped to 171/6 before Archer and Briggs added 51 for the 10th wicket.
“It went one way and then the other all day, really," said Head Coach Robert Croft.
"They won the first session and we did really well in the second to bounce back. And then I thought we would grab the day in the third, before that last wicket partnership.
“We would have liked to have finished them off sooner but we will come back tomorrow and see what we can do with the bat. The encouraging thing for us is that the ball didn’t do too much under lights.”