Cold smoking is natural progression for those hooked on grilling (usual start point), and then smoking (gateway drug!) to then finding ways of getting that hands-on approach to the same flavours that wood smoke imparts to food. We talk about flavour of food as being the primary reason for smoking, which it might be now but throughout our evolution was a side product of cooking our food over a wood burning fire which we have done for most of our history until the gas and electric ovens were invented in the early 20th Century. Smoking food also provides the ability to preserve it, and in lieu of cooking food (in the case of cold smoked salmon) reduces the ability of bacteria to spoil the food.
This time of year (winter) is ideal for cold smoking, especially more perishable items such as meat and fish, as they can be left outside to smoke where the temperatures will be less likely to mean the meat or fish will spoil by allowing bacterial growth.
Most of the change that happens to salmon in the process of turning it from raw salmon to smoked salmon occurs when the salmon is cured. Indeed you could, and people do, just cure the salmon in salt and sugar with no smoke added. Gravlax is one such food item, just plain cured salmon. There are many variations that can be applied to the curing process in terms of ingredients, time, proportion of salt and sugar (indeed types of sugar and salt). I have dabbled in adding juniper, bourbon, gin, pepper, maple and other ingredients over the years but in all honesty I can't say it's made much if any difference to the flavour. Indeed it is just another factor that risks it not quite turning out right.
Most of the knowledge I have gained and consistently lean on comes from coldsmoking.com, the lead for this site, Turan, is highly regarded by many in the barbecue world and have had the pleasure of seeing his cold smoking demo at festivals and competitions over the years. Indeed a quick chat after one of his demos gave me more insight on why he did certain things that I did differently and as such improved my understanding of how cold smoking works on meat and fish.
My method for cold smoking salmon is as follows: places the salmon skin side down on a thin layer of salt; lightly sprinkle the rest of the salmon with salt with more on the thicker parts; cover and cure for 12-24 hrs; rinse, pat dry and leave uncovered in fridge for another 24hrs (to allow sticky surface called 'pellicle' to form which allows the smoke a medium to attach to; then cold smoke for however long you need. My cold smoker will burn produce smoke for 14 plus hours, even as long as 16 or more, which for me is more than enough for salmon from a single smoke. If yours burns for less time then consider the airflow of the container you are smoking in, or if not enough smoke for another period.
You can, of course, cold smoke anything...well anything that will take on the smoke of which I have successfully tried olive oil, salt, garlic, chillis and cheese. The only kit required to cold smoke is something, usually referred to as a 'cold smoke generator'. Most variants of this device are a wire mesh, as shown above, filled with sawdust and a tealight placed at one end of the maze that will allow it to burn itself for long periods of time (mine, with the right (i.e. reduced) airflow will run for 16 or more hours). Many cold smoke generators will just need the sawdust smoking with a blow torch and will smoulder from there.