Hi Momstar:
I would suggest using cement (3 parts fine sand to 1 part portland) for your first layer; then using a tufa mix to bring it to the thickness you want. I use masonry sand and shake it through a sieve. Remember to shake the container after you mix the cement and allow it to rest for 5-10 minutes. This helps the air bubbles to escape, for a smoother surface. What a fun project to do with your kids! Good Luck,
'Lava'

Thanks Lava.

You didn't mention whether it was okay to inbed the saucer and if it would work. Could you expand on your comments? Sorry to be a pain.

I'm sure I will have fun. The "kids" are 15, 17, 19, and 21 yrs old. They learned a long time ago to keep the eye-rolling to a minimum and just go with it. They help me, I feed them. Everybody's happy.

What an interesting project. I wonder, what is the purpose of embedding the saucer?

But anyway I think you actually have the answer (almost) in the question. My suggestion would be to cast the bowl of the birdbath with a clear glass insert for the water. That takes away the pink, it is easy to clean, and will not threaten the birds.

If you completely imbed the saucer into the structure, you're still going to have a tufa surface, aren't you? I don't see any advantage to imbedding it. By covering the back (I'd cover it with a sheet of thin plastic first, for easier removal), you will maintain the symmetrical shape of the saucer. As well, you will still have your saucer to use for another one, should you wish to make more for gifts or for selling! I have a tufa bird bath made from a large rhubarb leaf. It's completely untreated with anything; it's about four years old and holds water quite well. I usually top it up every two or three days with fresh water.
Lava

I have used the plastic saucer for a large birdbath. For a really large birdbath, you can invert it and cover the outside. Or, you can line he inside and make a smaller birdbath. Anyway, all you have to do is cover the whole thing with a large plastic trash bag. I even use two or three for extra strength. I use equal parts of perlite, peat moss and portland cement. There's no need to embed the saucer. When it's ready to release, just pop it out and wirebrush. To waterproof, I just use Thompson's water sealer. Depending on the project, it may take two or three applications of the sealer. I've made many of these birdbaths, and they work well in the yard and the birds love them.

Since you are new - why not start off with small projects. Instead of a saucer - use a 12" plastic plate. It will be easier to lift when complete. Hypertufa can hold water. I have made bowl containers and left outside and rain filled them and remained in the container for days. Using two steps also works Fist layer with Portland and sand mix, then when cured for a few days, apply the hypertufa mix . The sand and portland mix is less porous.
As for sealers - there is much talk on other sites with using wax as a sealer - I am not sure how birds will react to chemical sealers.

If it were me - I would save the big project until after you have made several smaller projects. ---- Mike

Here is a link that might be useful: Some of my containers -