Try using a C-Clamp to apply pressure.
In this image I used C-Clamps to hold a CoB LED mounted on a small plate of copper to a copper water pipe.

I used CoB LEDs mounted to a rectangular bar.
This image is two 30 watt CoBs mounted on a copper bar.
I was testing conductive heat transfer and the influence of free convection.

You do not include enough pertinent information. The surface area between the iron and resistor and photo of your test setup would help. Specific type of ceramic and iron.

Try using a C-Clamp to apply pressure.
In this image I used C-Clamps to hold a CoB LED mounted on a small plate of copper to a copper water pipe.

I used CoB LEDs mounted to a rectangular bar.
This image is two 30 watt CoBs mounted on a copper bar.
I was testing conductive heat transfer and the influence of free convection.

I used CoB LEDs mounted to a rectangular bar.
This image is two 30 watt CoBs mounted on a copper bar.
I was testing conductive heat transfer and the influence of free convection.

Try using a C-Clamp to apply pressure.
In this image I used C-Clamps to hold a CoB LED mounted on a small plate of copper to a copper water pipe.

I used CoB LEDs mounted to a rectangular bar.
This image is two 30 watt CoBs mounted on a copper bar.
I was testing conductive heat transfer and the influence of free convection.