YES FOLKS - None to very little.
For classical chasing and repousse. The metal is bulged "up" from the back of the metal which results in the thinnest point being the highest point as seen from the front.
With high relief, he metal is pushed inwards and upwards to a bulge with a wee bit of sinking... and from there everything else is either pushed around or pushed down. This results in the thinnest point being around the edge of the design and the highest point, 9 times out of 10, being the original thickness of the metal. The Japanese also do their chasing work this way and it is called uchidashi and so if you are curious watch this video by Ford Hallam who I consider to also be THE MAN for Japanese metal work by a Westerner! Also Jim Kelso who is another great Japanese (western) metal worker has a great page about it.
I knew this was a great technique to learn so for example if (when) I do another tentacle, I won't break through when forming the suckers. Which is why I wanted to learn it and Heather being a master blacksmith/artist was just the person to learn it from.
After the lecture, we annealed our 18 gauge copper and started right in. My design was to be the shop pet (not shop elf) BOOTS. Actually she is the shop elf's cat. She spends most of the day following him around the house; naps on his lap and plays with him. Boots only see's me as the person who lets her into the house on weekend and who gives her the best chin scratches - other times she avoids me but I am gone all day with the day job so it is understandable that she has a stronger bond with the shop elf.
On the first day I completed the bulge and started pushing down Bootsies ears and eyes.
After class, we went to Heather's house for a dinner of hamburgers and great conversation. By 8pm I was exhausted so I returned to my AirBnB, took a hot shower and a long one too; then spoke to my sweetie and had a deep sleep - which was interrupted at around 3 am by the sound of gun shots through the open window. YES you read that correctly; I rolled over and went back to sleep.
On the second day I shaped Boots' cheeks, nose, lips and chin. I am almost done but I need/want to even out her lips, shape the nose a bit more and the left ear which I hope to do tomorrow.
Here are some process pictures as usual.