Four Mueller Brass employees were injured Friday night at about 10:45 p.m. when  a furnace failed then sparked a fire in the Port Huron casting shop  Runnels Street.  Of the four, two sustained burns, while the two others had smoke inhalation.  It is reported that all injuries were minor. Three of the employees were taken to an area hospital by Tri-Hospital EMS. Port Huron and Marysville firefighters extinguished small fires that were contained to the floor area beneath the furnace.  Reports say the fire began after a small explosion or mechanical failure. The cause is being investigated by officials at Mueller Brass