What material are good for mashing/boiling. I think stainless steel and copper are good, what about aluminum and enamel?
Both are fine, my pans are all aluminum as Stainless on that size is hard to find and expensive over here.
If you would cook on it you can brew on it.
Just be careful with what you use to clean them, John Palmer has some tips
I also want to share some points:
Both Stainless steel and copper have good conductivity, copper has a higher heat conduction. On other hand stainless steel has long life and stainless steel also has good corrosion resistance.
During CIP (Acid and base solution cleaning) of kettle stainless steel shows good response. These days the majority of food grade equipment is made of stainless steel.
One more point, Stainless steel is stronger than copper.
The most important consideration is cost, definitely stainless steel will be cheaper than copper.
I would strongly discourage aluminium. For frying pans it's acceptable, because it does not react with fats. For mash tun I wouldn't accept it. It reacts with acids easily. Especially in hot environment and over long time. It is only safe cold, for non-acidic foods. That's why all aluminium soda cans are lined inside. Anodized aluminium is better, because only hard oxide is in contact with food.
For details about drinks with aluminium, I advise reading http://www.who.int/water_sanitation_health/dwq/chemicals/en/aluminium.pdf and similar documents.
Behavioural impairment has been reported in laboratory animals exposed to soluble aluminium salts (e.g. lactate, chloride) in the diet or drinking-water in the absence of overt encephalopathy or neurohistopathology. Both rats (Commissaris et al., 1982; Thorne et al., 1987; Connor et al., 1988) and mice (Yen-Koo, 1992) have demonstrated such impairments at doses exceeding 200 mg of aluminium per kg of body weight per day. Although significant alterations in acquisition and retention of learned behaviour were documented, the possible role of organ damage (kidney, liver, immunological) due to aluminium was incompletely evaluated in these studies (WHO, 1997).
Copper has similar problems, but because it is far less common, accumulating significant dose is harder. Traditional copper cookware was covered with tin on the inside, because it was known since middle ages that not covered copper causes bad health when used daily.
Stainless steel is ideal, because it's hard to dissolve any traceable amount in water, and even if, some amount of iron, carbon and chrome are acceptable and even needed - I remember reading that most "first worlders" don't ingest suggested dose of chromium anyway.
Enamel is good, because it is inert. If damaged, enamel iron will just give iron to solution - flaw in beer, but not an health issue.