They’re pouring latex on him to make a false chest. So that they can place the arc reactor prop in him and make it legitimately look like it’s embedded in is flesh and with tears and scar tissue. The latex is colored so they can see where they are applying and how thick the layers are. It will then be airbrushed to his skintone and details like nipples scar tissue discoloration will be added.
Here’s that picture
Then the reactor prop is added to the dimple. Basically the latex becomes fake skin and they tear part of the center open to embed it.
Now you can see how realistic it looks.
I cannot express enough how much I love little behind the scenes things like this.
Ack no! That is silicon used to make the mold of his chest which is then cast and sculpted over and another mold made and then finally the (probably foam latex but could be silicon) appliance made.
The “dimple” pic is after a plaster jacket has been applied to keep the silicon stable- that’ll be what the white stuff is on his abs and arms. Also note how one of the technicians holds his head back? you need to be in just the right position to capture the same tendon and muscle tension you want in the final mold you are working over- my head cat is going to be denuded of ears as they are going to be in the way when I want to make cowls. That looks to be RDJs own musculature as all three techs are in the same positions it has to be the same day as the mold was made.
No tearing either! You can’t control tearing like that and you need precise multiple copies to shoot and reshoot scenes later… it’s so much more complicated. Lets give proper credit where due for the hundred of hours that go into making movie prosthetics and props. From design, to proof of concept to actual production to on set and warehouse maintenance.