SMA ENCOUNTERED ICING. CLB THROUGH ASSIGNED ALT; REFUSED CLRNC. DECLARED EMER.

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SMA X BEING VECTORED FOR APCH TO DENTON INBOUND FROM THE S. WX WAS A TEMP INVERSION WITH AN OVCST LAYER FROM 2000- 4200 FT MSL AND OAT OF 29-30 DEGS FAHRENHEIT. WE HAD BEEN ENRTE AT 6000 FT ABOVE THE CLOUDS WITH AN OAT OF ABOUT 43 DEGS FAHRENHEIT. APCH GAVE DSCNT TO 3000 FT; AT WHICH TIME WE BECAME IMC AND IN THE COLD AIR. WE BEGAN PICKING UP LIGHT MIXED ICE AND I NOTIFIED APCH AS SUCH. AFTER A FEW MIN; ICING BECAME WORSE AND IAS WAS DECAYING FROM 110 KTS TO 90 KTS UNDER FULL PWR. I ASKED FOR A CLB BACK TO WARM AIR AND WAS CLRED TO 4000 FT. AT 4000 FT WE WERE STILL IN IMC AND BELOW FREEZING. ICE WAS NOT ABATING. THE CLOUDS WERE VERY THIN ABOVE AND I EASED UP TO 4300 FT ON WHICH POINT WE BROKE OUT. APCH ADVISED WE WERE NOT CLRED TO THAT ALT AND TO DSND BECAUSE THEY HAD TFC ABOVE AT 5000 FT. I REFUSED AND SAID IF SHE WOULD NOT CLR ME HIGHER; I WOULD DECLARE AN EMER. APCH ANSWERED 'ARE YOU DECLARING AN EMER?' AND I SAID YES; AND CONTINUED ABOVE THE CLOUDS AND OUT OF THE ICE. I WAS HANDED OFF TO ANOTHER CTLR WHO CLRED ME TO 5000 FT AT MY REQUEST. AT THAT ALT OAT WAS 38 DEGS FAHRENHEIT. WE THAWED OUT AND LANDED AT DTO WITHOUT FURTHER INCIDENT. APCH WAS VERY RELUCTANT TO HELP WITH MY ICING PROB UNTIL I DECLARED AN EMER. ATC NEEDS TO TAKE LIGHT PLANE ICING VERY SERIOUSLY. I BELIEVE IT WOULD HAVE BEEN VERY HAZARDOUS TO FOLLOW INSTRUCTIONS AND DSND BACK TO ICING CONDITIONS WITH A 1/4 INCH LOAD OF ICE ACCUMULATION. DURING PREFLT WX BRIEFING; I SPECIFICALLY ASKED ABOUT ICING AND WAS TOLD THAT ICING AIRMETS WERE CANCELED AND THERE WERE NO PIREPS OF ICING IN THE AREA.