Encaustic medium sounds like “magic potion” to me, and in a way it is. Don’t you feel a little like the druid when you melt the wax? When you melt wax something extraordinary happens…
Encaustic medium thermal properties
Although you cannot see it, beeswax starts melting already at 40 º C and melting process ends at around 69 º C, during this process bees wax goes through distinct phases depending on which part of the wax is melting, some of them reach their melting peak at 72-73 ºC (pentaconane) whilst others at 52-53 ºC (Palmitate). Did you know that 70% of the wax is melted before wax reaches 42 ºC?
Please look at the thermogram above it is very interesting and we can learn a lot of what happens when we melt bees-wax.
- The curve shows what happens with beeswax’s temperature when we apply a constant flow of heat.
- horizontal axe shows temperature in º C, vertical ax heat flow (heat/time units)
- We start at 5 º C (probably they had the beeswax sample in the fridge!) and we start applying constant heat.
- at the beginning the temperature of the sample increases proportionally to the total heat we apply (horizontal line), this ratio is the Heat capacity of the material at the specific temperature range. Beeswax Heat capacity is 3,4 kj/kg (as reference water heat capacity is 4,1 kj/kg or 1 cal/ g)
- If we heat the sample a little bit more, temperature rises sharply, this meaning that now the material requires more heat than before to increase its temperature, its heat capacity rises. What happens with the extra heat we have to give? it is used to change the solid vitreal bonds to “softer” bonds and the wax starts melting.
- If heat continues increasing we arrive to another transition with a very high increase of heat capacity, the sample melts completely
- Once totally melted, wax starts rising its temperature approximately with the same ratio as at the beginning since heat is no longer used for melting.
Now you may be wondering what happens when the wax cools down, is it a reversible process?
The process is reversible but not completely, when we melt the wax part of its volatile components evaporate and the wax properties change with every single heating cycle, and remaining wax behaves differently. The curves above show how bees-wax behaves when heating and when cooling.
But what is viscosity?
viscosity measures the speed at which a fluid moves at an specific temperature.
Viscosity is not density. Just an example: oil floats in water for instance, this means that oil is lighter as water or that oil has lower density as water; but oil at the same time is more viscous than water. Just think how long it takes for a liquid to fall from a spoon, the longer it takes, the more viscosity it has.
Beeswax has a relatively high thermal expansion, specially between 40 to 50ºC (you have probably noticed it when you cast your cakes, when they cool they reduce their volume and you get a hollow surface…). This contraction when they cool makes it possible that our wax layers become smooth when cooling, since they stretch whilst contracting.
But you are probably now asking yourselves what on earth has all this got to do with encaustic? it has a lot to do and not only by melting the medium or fusing the wax layers, but also with the consistency of your wax.
Tips for encaustic practice
Melting encaustic medium:The heat flow should be gentle, don’t be tempted to increase the heat flow, the range of temperatures in which 70% of the wax melts is so narrow (4 º C) that the risk of over heating is very high. Once the wax becomes transparent and clear, reduce immediately the heat flow to the minimum. Bear in mind that in a temperature range from only 4 º C (from 39 ºC to 43 º C) 70% of the melting process takes place.
Fussing layers: Have you ever experienced to fuse an specific area until it “suddenly” melts completely and you get a totally uncontrolled spot that requires cooling before you continue? well, when we fuse after we have applied a wax layer, our panel can be around 40ºC, we have to apply a lot of heat to make it melt and get shiny, but once our wax layer reaches 70 ºC, the heat flow requirement plummets as in the graphic, and our wax layer suddenly becomes overheated. That is what happens in our overheated spot, the only way to avoid it is being patient applying always a moderate heat flow .
Damar resin: As is shown in the figure above, beeswax viscosity is highly temperature sensitive. The most important reason why we add resin to our medium is to assure the right consistency of the wax at working temperatures. Resin makes the beeswax more workable for our purposes.
Extra tip: If we pre-warm our panels before we start applying wax we will be able to extend the wax more easily, the aim behind it is to allow us a gentle stroke, keeping wax low’s viscosity the maximum time.
Your brush stroke: As explained above, the consistency of melted beeswax is determined by its viscosity, when temperature increases, the viscosity decreases. Viscosity is therefore a decisive parameter when extending your wax stroke in your panel, it is responsible of the amount of wax your brush is able to withhold, and the thickness of the coating in your panel. Temperature control is of paramount importance to the resulting surface. If your wax is too warm, your brush will retain very little amount of wax, and will keep dropping until it touches the panel. If the wax is too cold your brush will retain enough wax, will not drop, but the wax might be too “thick” when it reaches the panel. So you have to get the feeling for what better fits you. Each wax behaves differently so, if you don’t want surprises, keep using the same wax.
Bubble formation: And bubble formation? bubbles are trapped air encapsulated in wax. This happens because your ground is porous and the outer surface of your layer starts solidifying before the rest. Some superficial bubbles will disappear while fussing. Try pre-warming your panel and try applying thin layers, these should help.
This post is also available in: Spanish