As winter approaches it is not uncommon to see chimera and regular African violets being shipped using 72 hour heat packs. But to do it safely there are many misconceptions that must be cleared up as to what the packs can and cannot do.
The heat packs most commonly used are made up of sawdust, iron fillings, salt and water. They come sealed in a cellophane type outer bag in which the heat pack resides. Upon opening the cellophane bag it automatically becomes activated when exposed to air (oxygen to be specific). As the iron powder comes in contact with the oxygen in the air and the moisture (water) in the sawdust, it causes the iron to lose its metallic properties (that is called corrosion) and it gets reddish brown, hardens and flakes (turns to rust). And when something rusts, it gives off heat. We usually don’t notice the heat in rusting objects since we are not exposed to a mass of rust at one time with a large surface area unless you are talking about a car I once owned. But in the context of a 1/2 lb of iron power used for 72 hr shipping, we are talking a large iron surface area and a lot of rust occurring in a small area, so measurable heat is generated.
These packs work well above freezing. For when the packs are kept at freezing for an extended period the water in the packs freeze and the reaction stops and hence so does the heat. Below is a reaction measured for 120 hours at one minute intervals. The measurements were made at room temperature of 70 degrees F.
You can tell when the heat pack is spent, becoming very hard and brick like. Two heat packs are shown. The one on the left was just removed from it’s cellophane wrapper and the contents when the bag was cut open is soft, black in color and frankly something you might feel is appropriate to pot your plants in. The bag on the right is spent. Actually the concave surface is where I placed the temperature probe. You can see the contents are hard and fully rusted.
Heat packs used in the context of taking the edge off of cold weather shipment and not as the sole heat source in an uninsulated container in subzero weather will minimize any disappointment when your plants arrive.