This artist uses this method, and the link to this page is for her book which teaches the method. The link at the bottom there is to a mosaic site that sells the mastic and describes its use. http://web.me.com/laurelskye/Laurel_Skye_Designs/MY_BOOK_is_here%21.html

Here is a link that might be useful: mastic

What Laurel is describing is IMO - the ready-mix mortar for mosaics. Lowes carries in. I used it when I was a beginner, and while it w/work, it does become brittle as she says. Waschbear on Flickr uses the push/sqush method, which you want to do, and I have used it recently and love it. I use Mapei Ultra Flex II. Mapei carries the ready-mix. Don't know what Waschbear uses. She's very gracious about answering questions. Posting a link to my favorite box on which she uses the technique. Ultra Flex II comes in white and gray, and you can color it w/colorants.
Waschbear's White Box

BTW - I forgot to mention that in spite of what Laurel said, my ready-mix mildewed very quickly. I had bought this mixture a couple times, and both times, the mildew appeared b/f the tub was half used.

Oh boy! Sensory overload.
Waschbear is the one I saw on etsy. How dow you keep it from getting dry and hard before you are done? Her colors are beautiful.
Steampunk bathroom. I'd like one if I didn't have to clean all those nooks and crannies.

You have to work in sections-and you have a fairly decent working time with it. But you butter up a section, fill it and then do another.

I mix up about a cup and a half of mortar to a peanut butter consistency. Depending on the humidity, I have an hour or two to use it. Depending on the size of the tess, I put a big blob on the tess and stick it as close to the other pieces as possible. You want the mortar to push up so that the adhesion to the tess you're placing down and to those pieces of tess around it as possible. You can overlap pieces, add teensy beads to the mortar that pushes up so it's not so noticeable. I get so carried away w/additions and coverage I don't know when to stop. I asked Waschbear for permission to make a white box from inspiration by looking at hers. Of course, she was gracious enough to say she didn't have a problem w/that. I made the box, a small mirror and a vase. Look in my "Gifts" album, from which I posted my present shrine project, to see the mirror I recently completed for my DD. Of course, after I posted a picture, I added more to it. It's so addicting. LOVE the push/sqush. Try it on a small box. You'll love it, and there's no grouting. It self-grouts. I hate to grout, and this is my new favorite technique on some things.

I think the "mortar" I have been using is mastic. The man at Lowe's said it was mortar. The bucket says adhesive. Live and learn.
It was my first purchase and it has surely lasted. Premixed in a bucket. This is the stuff I was using that would come loose occasionally when the moisture of grouting got to it.
I shall have to try this.
I ordered the mosaic book by Laurel Skye.

Anything premixed is mastic. You cannot premix mortar because once its mixed there is no way to stop the setting of it. I did find some interesting info on concrete that would apply to mortar as well. I work in an unheated garage so it is often cold. working in cool temperatures slows the set of mortar and therefore increases its strength TO A POINT. The chemical reaction requires a certain temperature, and once the actual cement drops below this temperature, it will set but the chemical reaction that strengthens it stops and it will crumble. This is why bridgework etc stops when its cold. This is around 30-40 degrees. So if you are working with mortar in the cold keep this in mind. Until the mortar is set, it shouldnt be allowed to drop below 40 degrees. setting time doubles for every 10 degree drop in temperature below 70 degrees. I would hazard a guess this might apply to grout also.

Sine grouting is my very least favorite, mastic sounds good to me.