The basic idea is that you grind up pigment and mix it with egg yolks whipped up with a bit of water. We used colored chalk, but in the past, artists would use mineral, plants, and even insects.
The grinding was much harder work that we thought. This led into discussions about how useful the workshop/apprentice model was in the past when artists were responsible for manufacturing their own paints, brushes and other tools.
We used shells and rocks to grind and immediately saw how hard it was to get a perfect, uniform powder.
My son entertained us with stories of Leonardo Da Vinci and his many failed painting projects. My daughter decided to try to use just the egg wash to make a glaze. When the paintings were done (on paper and wood) and dry - it was amazing to see a completely different sheen than any other paint we have used.
Later in the day, we talked about how learning about egg tempra might inform other experiments and listed the possible applications of our new skill, including using it for painting flying machines.
While we painted, we listened to Palestrina, the Renaissance composer who came a few hundred years later and made music more three dimensional, just like Giotto did with human figures. The choral music was so relaxing and uplifting we listened to it all day.
On a side note, I just finished reading The Girl With a Pearl Earring by Tracy Chevalier and really enjoyed it. It is based on the Dutch painter Vermeer and his maid and their relationship. One of her duties is to grind pigment and mix paint and the rich imagery from the novel were running through my head as I did this project with my children today. If this is your sort of thing - check it out. The movie based on the novel is good too.