A PA-60 PILOT ENCOUNTERED ICING CONDITIONS IN THE CLIMB. HE DECLARED AN EMERGENCY AND DESCENDED TO A LOWER; WARMER ALTITUDE.

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WHILE ON A RADAR VECTOR AT 12000 FT MSL IN GOOD VFR CONDITIONS; I NOTICED A LARGE CLOUD FORMATION APPROX 15 MI DISTANT WITH TOPS SLIGHTLY ABOVE 12000 FT. TO AVOID A POTENTIAL ICE ENCOUNTER; I REQUESTED A HIGHER ALT. I WAS TOLD BY ATC TO STAND BY. I WAS NOT TOLD THAT GRANTING THE REQUEST WOULD BE DELAYED FOR EITHER A PERIOD OF TIME OR A PARTICULAR DISTANCE. WHILE AWAITING THE HIGHER ALT CLRNC; THE ACFT ENTERED THE CLOUD FORMATION JUST BELOW THE 12500 FT TOPS. I SOON OBSERVED RIME ACCUMULATION; NOTIFIED ATC THAT I WAS ACCUMULATING ICE; AND REQUESTED IMMEDIATE AUTH FOR THE PREVIOUSLY REQUESTED HIGHER ALT. I WAS AGAIN TOLD TO WAIT. SOME TIME LATER; ATC GRANTED THE HIGHER ALT CLRNC. I GENTLY PULLED THE COLUMN TO INITIATE THE CLB AND IMMEDIATELY EXPERIENCED AN AIRFRAME BUFFET. THE AIRSPD INDICATOR INDICATED THAT AIRSPD HAD DECLINED FROM THE USUAL 160 TO 120 AND WAS DETERIORATING FURTHER. I CONCLUDED THAT ICE ACCUMULATION HAD IMPAIRED THE ACFT'S FLT CHARACTERISTICS AND NOTIFIED ATC THAT A CLB WAS NO LONGER POSSIBLE. I REQUESTED AN IMMEDIATE DSCNT TO ESCAPE THE ICE BUT WAS DENIED THAT CLRNC DUE TO TFC CONFLICTS. I DECLARED AN EMER; ADVISED ATC THAT I WAS DEVIATING L TO LOCATE CLR AIR; REQUESTED A VECTOR TO CLR AIR; AND A LOWER ALT ASAP. I SUBSEQUENTLY FOUND CLR AIR. ATC THEN ISSUED A VECTOR WHICH I REJECTED ON THE BASIS THAT IT WOULD HAVE DIRECTED THE ACFT BACK INTO THE CLOUDS. I SUBSEQUENTLY GAINED VISIBILITY DOWN TO THE GND; SO INFORMED ATC; AND ADVISED THAT I COULD ACCEPT 5000 FT MSL ON ANY HDG. I WAS THEN GRANTED 5000 FT AND TOLD TO FLY DIRECT ZZZ. I DSNDED PROMPTLY TO 5000 FT AND THE ICE FELL AWAY IMMEDIATELY. THE INCIDENT WAS CAUSED BY ATC'S FAILURE TO EITHER PROMPTLY GRANT MY HIGHER ALT CLRNC; OR TO ADVISE PROMPTLY THAT GRANTING IT WOULD BE DELAYED. HAD I BEEN ALLOWED TO CLB IMMEDIATELY; NO CLOUD PENETRATION WOULD HAVE OCCURRED AND NO ICE WOULD HAVE BEEN ENCOUNTERED. HAD I BEEN ADVISED THAT THE HIGHER-ALT CLRNC WOULD BE DELAYED; I COULD AND WOULD HAVE REQUESTED A) A HOLD AT ALT; B) A DEV L OR R; C) A 180 DEG TURN AT ALT; D) VFR ON TOP; OR E) CANCELED IFR. ALL WERE OPTIONS AVAILABLE AT THE TIME OF REQUEST; BUT NONE WERE PURSUED DUE TO ATC'S APPARENT INTENTION TO GRANT THE HIGHER-ALT REQUEST. BY THE TIME THE HIGHER ALT CLRNC WAS GRANTED; NONE OF THE FOREGOING REMAINED VIABLE OPTIONS. IN THE FUTURE; I WILL BE MUCH LESS DEFERENTIAL TO ATC ABOUT SECURING CLRNC AMENDMENTS TO AVOID HAZARDOUS WX. I WILL REQUEST SUCH CLRNCS SOONER; AND WILL BE MUCH MORE AGGRESSIVE ABOUT EITHER OBTAINING THEM OR NEGOTIATING/DEMANDING A SUITABLE ALTERNATIVE.