AN ACR MLG HAD PRESSURIZATION PROBS THAT REQUIRED AN IMMEDIATE DSCNT.

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WE HAD CABIN PRESSURE WARNING LIGHT AND CABIN SLOWLY CLBED TO 10000 FT. EARLIER; WHILE CRUISING AT FL350; ENG EPR GAUGES CLBED TOWARD TOP OF SCALE. (WE WERE IMC AND RAT TEMP APPROX MINUS 20 DEG C.) TURNED ON ENG ANTI-ICE AND THAT CORRECTED IT. FO WAS FLYING AND HE WAS CONCERNED OF WING ICE SO WE TURNED ON AIRFOIL ANTI-ICE. WE DID NOT APPEAR TO HAVE ANY WING ICE. WE SHOULD HAVE TURNED IT OFF. WE HAD SEVERE TSTMS ACROSS MN; DOING A LOT OF DEVIATING; LIGHT TO MODERATE TURB. ST ELMO'S FIRE AND WORKLOAD WAS VERY HVY. CABIN ALT CLBED VERY SLOWLY (APPROX 100 FPM) AND WE DIDN'T FEEL ANY PRESSURE BUMPS. WHEN CABIN PRESSURE WARNING LIGHT CAME ON; WE DONNED OXYGEN MASKS AND I KNEW RIGHT AWAY THE AIRFOIL ICE CAUSED THE CABIN PRESSURE TO CLB. I SHUT THE AIRFOIL ICE OFF AND CLOSED THE XFEEDS. THEN I ACCOMPLISHED FIRST ITEMS ON THE LOSS OF CABIN PRESSURE CHKLIST. WE STARTED DSCNT FROM FL350 AND NOTIFIED ATC ASAP WE HAD A CABIN PRESSURE PROB. WE VACATED FL350 AND BEFORE ATC CLRED US FOR LOWER; WE WERE DSNDING THROUGH APPROX FL340. THEY CLRED US TO 8000 FT. THE CABIN PRESSURE STARTED DSNDING RIGHT AWAY AND DID NOT GET ABOVE 10500 FT. CABIN OXYGEN MASKS DID NOT DEPLOY. A COUPLE MINS LATER; ATC ASKED US IF WE NEEDED TO DECLARE AN EMER OR NEEDED ASSISTANCE. CABIN ALT WAS DSNDING; WE SAID NEGATIVE AND CABIN PRESSURE WARNING LIGHT WENT OUT SHORTLY AFTER THAT. WE WERE OFF AIRWAYS IN REMOTE AREA OF NORTHERN MN. NO TFC IN AREA AND ATC DIDN'T SOUND CONCERNED ABOUT US LEAVING FL350. WE LEVELED AT FL290 AND CONTINUED TO DEST WITHOUT FURTHER INCIDENT.