A patient 69 from Nick Selman helped Glamorgan restore their advantage against Northamptonshire with the Welsh county leading by 248 runs at the end of day three.
It was Selman's first half-century of the season, with the Australian putting on 100 for the third wicket with Billy Root (56) to put Glamorgan in a strong position heading into the final day.
"It was good to get a few since I haven't scored the runs I would have liked, but I haven't felt too much out of touch and the wicket was good to bat on and get into a rhythm," said Selman. "Hopefully I can build on that for the remainder of the season.
"It's a tough wicket to score quickly on, it's very slow and they bowled well, but it was about keeping in my mind that it was going to get easier and I managed to get a few away at the end.
"We'll look to bat positively and see where we're at. If we can hit the top of the stumps when we bowl, there could be a chance of taking ten poles. The ball's in our court as to when we do declare which is a great position to be in."