Effective purging is dependent on three things – temperature, surface area, and pressure.
The right combination of these parameters will remove butane from the extract and leave you with a translucent slab of high quality oil. The wrong combination will leave you with a sticky mess that’s destined for a burn pile in your backyard.
To preserve terpenes, the lower the temperature, the better.
Butane and terpenes are some of the first molecules to evaporate while purging. An important point on purging temperatures: vacuum oven temperature may not translate to oil temperature. This is a point that is easily forgotten, and can be the difference between a product that shatters versus a pull-and-snap consistency. Check the temperatures of your oil with an infra-red thermometer.
There are several ways of removing the butane while retaining terpenes.
Keep in mind the boiling point of butane – it’s -0.5C/32.5F. If you purge long enough at room temperature at full vacuum, you will boil off all the butane – that process can take up to 5 days. Alternatively, you can increase temperature on your vacuum oven until the oil reads a steady 46C/115F, then pull a full vacuum.
Spread the extract as thin as possible so that the butane is exposed to the surface and boils off with minimal effort.
Surface area is the next big point in extractions. You need to spread the extract as thin as possible so that the butane is exposed to the surface and boils off with minimal effort. If you have the extract pooled up in a small container, it will eventually fully purge, but at geological time scales. This is best done with a two step process of pouring out the bulk of the extract onto one silicon mat and scraping out the residual extract with a spatula followed by spreading it onto a second silicon mat. This process minimizes auto buttering of the poured batch and requires the least amount of handling.
Cycle your vacuum on periodically.
Vacuum cycling is the next issue: cycle your vacuum on and off every 60 minutes, and keep it on for at least 10 minutes. Whenever there is a vacuum on the chamber, the extract is off-gassing butane, terpenes, and any other volatile molecules. If you pull vacuum on the chamber, but then shut off the pump, the chamber will fill with butane. Without getting too technical about the partial pressures of gasses, the butane gas in the chamber affects the rate of evaporation from the extract; it’s like having a room filled with balloons – if you’ve filled up the room, you can’t inflate more balloons until you get rid of the ones occupying the space.
By controlling the purge temperature at the oil, increasing the surface area, and cycling your vacuum, you’ll be on your way to dialing in your strain’s preferred conditions for making exquisite extracts. Unfortunately not all strains behave the same, and you’ll have to adjust temperatures and the timing of your vacuum pulls accordingly.