NOT clearing screen

In one trail we have two stims - an image and a scale. They apear at that same time (compound screens, no problem!). Then the image has to disappear and we need a black screen during 1000 to 1200 ms (this interval is not a problem) but we want to keep the scale so actually the screen won´t be completly black. After those 1000 to 1200 ms we want the next trial to start. So in the outcome we want the scale to be on the screen during the whole experiment. How do we do that? We now defined a black square on the black screen as a stim that appears during the defined period of time, without clearing the previous screen, but this does not work: The scale disappears.
I should add, maybe this is the problem, that we defined 1000-1200 ms for the last stim of the trial (the screen without the picture, but only the scale). I suppose that after those 1200 ms the screen is automatically cleared before the next trial begins. But we don´t want the scale to disappear and re-appear with the next picture, but to stay where it is...

What should we do?

Thanks for your suggestions!

Best wishes,
Nadine