Source: Rise of New Thassilon, pg(s). 84
Early in their undeath, ashen forgemasters resemble living salamanders. As time passes, their flames dull and their skin loses colour, until the ashen forgemaster looks like a statue; despite this, the heat of the Plane of Fire continues to burn. As their body decays, ashen forgemasters collect the remains and mix them with liquid metal to rebuild themselves. An ashen forgemaster stands between 9 and 15 feet tall and weighs around 5,000 pounds.
When a salamander dies after having previously been bound to a forge and tasked to toil by its slavers, the binding magic forces it to continue its work, raising it from death as an ashen forgemaster.
As long as the forge to which it is bound burns, an ashen forgemaster cannot be destroyed and will automatically rejuvenate after being 'killed'. The binding magic also connects the forge to the Plane of Fire, meaning that a forgemaster cannot free itself by destroying the forge, and that extraordinary water or ice magic is required to douse the forge so the forgemaster can remain dead. A single forge can only host a single salamander, but it is possible to link multiple forces to simultaneously enslave multiple salamanders.
Ashen forgemasters spend centuries in isolation, only interacting with their captors, who rarely see them as anything more than tools and might ignore them for years at a time. Their sanity erodes, and they become violent, sadistic, and paranoid, lashing out against anyone they perceive as thieves or distractions. A few forgemasters who are not abandoned or whose task is actually achievable might retain their sense of self, and are single-minded in seeking freedom, up to and including making deals with the salamanders' hated efreeti enemies.
Ashen forgemasters bound to nearby forges often form close alliances. In order to ground themselves in reality, they surround themselves in their products, which grow weirder and weirder as their insanity grows. Some chain themselves to their forges to represent their slavery and then measure the distance they can safely stay away from their forge.