Covering window frame gaps with red clay

The windows have quite big gaps on the sides to fit the window in. Although the gaps are within the acceptable range of 5 cm these gaps don't look pretty. The windows there fitted in with screws and expanding foam and the gaps are filled with the foam. When cutting the foam, it remains visible. We covered this with red clay, which seemed the most suitable colour to match the window frames. The video doesn't show well the quantities as it was a bit of experimentation at the start. We now use 1 clay:1 sawdust:3sand and that works well without giving big cracks. We finish it with a layer of linseed oil.