I think you are very close to getting this to work.
If you are using FileMaker 10 or 11, you can set an OnLayoutLoad or OnRecordLoad trigger to run your script to select the appropriate layout either each time you go to a layout or each time you go to a different record.
You might be able to avoid multiple layouts altogether. If the main difference between layouts is one or more graphics that "show the chemistry", you could store those graphics in container fields in a related record. Then a single layout might work for all your metal types by displaying the appropriate graphics from the related "metal type" record.
Which script trigger would I use if I wanted it to go off when I chose a value from a drop down list?
With Drop Down lists, OnObject Save and OnObject Exit work. (OnObject Save fires only if you change the value).
With pop-up menus and radio button fields, On ObjectModify is a good choice.
Thanks alot, that worked perfectly.