All of the mask pieces starts off as sculptures in oil-based clay. Since I didn't have a cast of my customer's head I used a female porcelain wig stand to sculpt on, and crossed my fingers the fit would be OK. For the shoulder and collar bone pieces I'm using a hard polyfoam bust bought many years ago from monstermakers.com
My favorite plaster is Ultracal 30. It's very hard and duplicates small details very well. After the first layer, which is brushed on, I slap on another layer with burlap dipped in plaster.
The burlap reinforces the plaster, preventing it from cracking when the mold is pryed off the sculpture. After the mold is removed the clay is cleaned out. Liquid latex is sponged into the mold, making sure airbubbles are avoided (as best one can). Using a sponge helps make the edge of the mask piece very thin, which is something you want when you glue it to your face. The latex piece is dusted with talctum powder (or corn starch) before being removed. If you don't do that the inside of the latex piece will stick to itself.
My friend Liz acted as a makeup model, very patiently letting me stick scaly rubber pieces all over her face and down her cleavage. I decided to color all the pieces with a neutral human skin tone by mixing latex and pigments before casting the latex into the molds. I did this because at this time I was practically out of the ingredients for making "PAX paint"; a very strong makeup base made with ProsAide glue and acrylic paint. After having applied the mask pieces I would've covered both Liz´skin and the latex pieces with PAX. The forehead piece is not yet trimmed in this photo.
Using the little ProsAide I did have I made some "bondo"; a paste mixed from Cabosil powder and ProsAide. I covered all the transitions between mask and skin with the bondo, making the mask edges practically disappear. This little trick helps selling the illusion better. In this photo I've applied the bondo to the nose and chin pieces.
So, here's the finished mask job with the Cardassian pale greyish yellow skin tone added in Photoshop. A bit of cheating, I know, but this at least shows how the mask will look when applied and painted properly. Having cast the mask pieces in human skin hue helped make the Photoshop tweaking easy. And that's the Cardassian insignia in the background.
While playing around with glue and rubber I also made Liz up as a Vulcan, using ears that are usually created for Elven LARPers. The ends of her eyebrows are covered with wax and bondo, with makeup powder added. You can get a VERY smooth finish using bondo, if you don't ladle it on to thick. Just wet your index finger in water and lightly rub over the bondo paste to smooth it out.
I glued down the latex pieces using spirit gum, but ProsAide is a better adhesive. Do remember to use a proper remover, though. Bond-Off is a really good one. All the latex mask pieces worked fine, especially those stuck to Liz´face. The neck pieces were the only worry, buckling and wrinkling in a weird-looking way when Liz raised her shoulders. So back to the drawing board with those. I added soft polyfoam to the neck pieces, which helped.
But all in all, mask pieces made in liquid latex need not be a horror to wear. Latex is a good economical choice compared to foam latex and silicone and it's up to each and every person to decide if it's something that would be OK wearing during a day at a con.
This image is really a cheat, because it was created in Photoshop, but it shows Liz in makeup as a Star Trek: Original series Klingon warrior woman. It's a design experiment to see if it would be a good idea to make a fan film with her in this getup. It'll probably happen some time.
One last image: I eventually made up my friend Rolf (sadly gone now) as a Tellarite ambassador. I wish I would've filmed him like this and saved the footage for some future project.