I found this fun idea on pinkandgreenmama.blogspot.com and tried it out last year. Its highly adaptable to a variety of ages, but I use it with my 6th graders and they love it. This year, we tried it with a holiday theme. We talked a little bit about stained glass windows and how the lead lines in the stained glass not only hold the glass pieces together, but also add a design element to the piece. We mimic that with the black glue, which provides a striking contrast to the bright painting.
We brainstormed winter and holiday themed ideas on the board to give students lots of options and inspiration.
I also encouraged them to do a heavy wash of whatever color they were using, then use a little paper towel to dab at the middle of the space so the center of each area would "glow." (Some students used this more than others.) Overall, a great project to use to work on composition and craftsmanship! Happy Holidays!
(**Note: This year I tried using tempera in the glue instead of acrylic paint... stick with the acrylic! It "puffs up" better!) :-)