I know this is a zombie thread, but I just wanted to share my experiences with others about this topic, since when you Google this (sticking or glueing MP to CP), not much info comes up (actually none at all... The only info that comes up is embedding MP into freshly poured CP- ie: not applicable), and this thread, which is the "holy grail" of actually attaching MP soap pieces directly onto hardened bars of CP soap. I'm hoping maybe others who are trying to find an answer will find some helpful info here, even if this thread was started years ago.
Like OP, I too wanted to attach a cameo piece of soap (made out of MP) onto the face of a bar of CP soap. I had a problem tho... My soap was a loaf, so I couldn't attach the MP cameo piece until after my CP soap was cut. I decided I'd do a test batch... Just something small I could play around with & it wouldn't be a total loss if my experimenting came back unsuccessful (or if I messed up the soaps in the process!).
Here's what I found: MP soap just doesn't want to stick, or adhere to CP. I made my MP cameos, I made my CP soap (and cut the bars), and then I melted down some MP base to act as "glue". The MP cameos surely did stick to my CP bars (and I even scored both the backside of the MP embeds and the CP bar itself), and it stuck... For a little bit. The tiniest effort to pull the MP piece off, resulted in that very thing happening... It would FEEL firmly attached, and STAY attached, but a simple prying with my fingers would pop that MP embed piece right off again. And the melted MP soap I had put down as "glue" on top of my CP bar would peel right off too, as if it had never been there. It was frustrating because it was imperative that these MP cameo pieces STAY on the bars throughout use, not just to be temporarily decorative (ie: I didn't want my MP cameo pieces popping off in my customer's hands).
So then I thought, "Well, I just won't TRY to pull them off. I'll be gentle & they'll stay put, and surely the first time someone gets these bars wet, that MP cameo piece will stay affixed to the CP soap!"... I was WRONG! As my CP bars cured & lost water weight, the bars naturally shrunk down in size, and when this happened, I slowly watched many of my "glued" MP cameo pieces pop right off on their own.
Ok, so that wasn't the solution, but there HAD to be one! MP just doesn't want to stick to CP for the long haul, so I thought I'd just make my cameo embeds with CP instead... That was a nightmare! My tiny, intricate little cameo mold just isn't made for CP. Didn't matter how fluid my batter was, if I wanted the tiny intricacies of the mold to be imparted to soap, it had to be MP, which put me back at square one: How to affix these MP pieces to cut CP soap bars & make them STAY PUT!
My next attempt was rubbing alcohol and that turned out being a fast "NOPE!". I saturated both my CP bars & MP cameos well with RA (not too much, but just enough), and after a bit of slip-sliding around, those MP cameos seemed to have stuck!... Until the alcohol evaporated & then they just popped right off again. *SIGH*.
So what was next?? I thought about whipping up a quick batch of CP soap frosting, putting a small dollup of that on the back of my MP embed piece & sticking it to the bar that way, but as a last ditch effort to try to find an easier way (and a way that would leave my MP piece sitting completely flush on the face of my CP bar), I decided to give distilled water a try.
I saturated the back of my MP cameo pieces with as much water as I could without affecting any of the design on the front, and then used a soft bristle paint brush to wetten the face of my CP bar too, but only in the spot where I'd be laying down the MP cameo piece. Just like the RA, there was a little bit of slip-sliding around, so I just gently held the MP piece to the face of my CP soap, just to the point where it would stay there on its own, and then I left it completely be. If I continued to futz-around with it, it wasn't going to ever stay put, and if I sat there applying pressure, I risked ruining the face of my MP soap pieces and/or my CP soap. So a tiny bit of light pressure, just until the MP cameos weren't sliding around, and would stick on their own, and then I left them completely be for about 12hrs.
When I came back to check if my MP cameos were going to stay put on my CP soaps, a hammer & chisel would've been needed to get these suckers to come apart!! THEY WERE FIRMLY ATTACHED TO MY SOAPS & NO AMOUNT OF PRYING WAS GONNA POP THEM OFF!!! As my CP soaps cured, the bond only became stronger, and when I tested a bar in the shower, the MP piece never came off! It had become firmly welded to the CP bar!
So from a lady who spent weeks upon weeks of frustrating experimenting to find the best, most lasting way to get MP soap pieces to STAY adhered to hardened CP soap bars, I can happily share that distilled water is the way to go!
I hope this helps someone out there because it's info I would've loved to have found myself, but that's ok because it ended up being a worth-it experience, and if I can help just ONE person, it makes it that much more so! Happy soaping everyone!
