This Morning's Frost
Thanks for your comments,
Augustin Man, brianj, filibuster, PhotoRaw, MurryG, VisionLight, Jim B, Patricia H Cate, evancj and grcolts.
I am lucky to have a garage with windows on two sides. One window gets little sunshine, so the crystals there have massed together into an uninteresting glob. The other window sometimes gets several hours of afternoon sun, so the crystals often vanish altogether and the whole process starts afresh. I carefully cleaned the window to minimize blemishes in the photos.
I tried taking some shots at night by shining a light on the crystals. Results looked promising but some parts of crystals were too dark, while other parts had blown highlights. There was also a shadow image on the back pane of glass from the double glazing. Next, my loving wife agreed to stand outside in a snowdrift with a light, while I remained in the relative comfort of the garage to take pictures. There were still problems with too little and too much light, depending on the orientation of the crystal parts. So daylight seems to work best for me. With the light balance on "cloudy" the shots are a pleasant brown, but not what you expect, so I have been using the "tungsten" setting to get a blue tinge, which I strengthen in post-processing.
Thanks again for your kind comments.