You probably want to go with ingots of whatever type, steel is more valuable than iron. The reason to go with ingots is for each thing you melt/make you get pps so if you are working with stocks you need a lot more ore for the same amount of practice. Combine toy coal and iron and then finish the steel or coal sludges individually, you can work on as many at a time as there are slots available in the furnace. If you leave the sludges in the stack of ten to complete melting you get on practice point for the whole stack, if you separate them you get one for each or ten total. The same thing for casting if you cast a stock you get one practice point if you cast ten ingots you get ten practice points.
Just remember that as your skill advances what you get practice for changes, new task become available and easier tasks stop giving practice. Keep an eye on the status tab.
If you plan on making weapons then you need to make sure you have the right combination of ingots and/or stock. Steel is what all weapons are made from, as far as I know, and bronze and gold are used as well as steel in shields.
