I need to grow a 3.25 foot wide salt crystal and don't know how Answered

What's the purpose of this big lens?

If you make a single salt crystal that wide, you will be making a world record-breaker.

If you are wanting to experiment with making lenses, and have access to a kiln, I would work with glass, because it is much less fragile than sodium chloride, and, much more importantly, it will not absorb moisture from the air around it.

Skylights are made by heating plexi till it's floppy, then inflating it. Once cooled it is domed.

To make your lens, do this with two circles. glue them together and fill with water.

The only affordable lens of that size that I can think of is a lens made out of water (or a suitable oil), comtained in clear plastic that's suspended in a frame.
The obvious disadvantage is, of course, that your lens can only be used in a horizontal position.

Big lenses COST. I had a quote for a telescope project with some special optics "only " 10" in diameter a couple of weeks ago - they are 15,000 each.

3.25 FEET is beyond the state of the affordable art in ANY material !

Steve

I dont think a crystal over a meter in diameter is something you can make at home. Closest I could find was this: https://lasers.llnl.gov/education/fusion_fun/recipes/crystals.php

The trick to growing crystals is control of every parameter of the crystallization process and time. For such a big crystal you would need very fine control of all parameters and a looooong amount of time.

I do wish you good luck though and if you find a method to do it please share!