is the yacht varnish fireproof?

i'm just restoring a slate fireplace which has a lovely pattern on it but i'm guessing it's been varnished in the past but it's been stored in a garage and the varnish has gone milky making it look pale. when i've collected it from the seller he had it out in the rain,
1, it made the fireplace look a lot darker and greatly improved it's looks.
2, it also made the varnish soft so it came off with with a
good rub down with a cloth and hose pipe,

the result though has left the paintwork dull and pale but it looks a lot better when wet so i'm guessing using varnish would restore it's looks but it will be around a working logburner so obviously it needs to withstand the heat.
any advice would be appreciated.