Heat Treating Minerals

All about Heat Treating

Cartoon of Gem Crystal of Tazanaite being turned from brown into blue by heat

Is it hot in here or am I just becoming more valuable?

 

Color changes in minerals and gemstones can occur by heat, uv light, x-rays, electron beams, protons, neutrons, deuterons, alpha, beta and gamma radiation.  However, due to the cost in producing these effects, only heat treating is the most universally used.

In many cases, minerals can be treated in something as simple as your kitchen stove, however, most work is done in a kiln with a controlled setting, which is a common accessory in many craft studios.

We suggest a digital kiln for your heat treating needs.  In any good preparation studio, a kiln like one available from Paragon, like the SC2.  Priced in the $700 range, this workhorse unit is pefect for crystals and gems.  Large stones tend to break apart while being heat treated, so the material to be heated will either be a finished gemstone, or one that has already been cobbled down.   Large crystals are rare to have survive and have a universal treatment.  A nice compact unit is a perfect accessory to an workstation.

Below we will tell you about different minerals and what to expect when heat treating them.

Andalusite of gem quality has been shown to change to a pink hue when heat treated.   The addition of copper makes this material have specks of bright red inside.

Angelsite loses color when heated.

Anhydrite with color can be bleached quickly by heating.

Apatite is most often turned colorless after heating, however, the deep green crystals can turn a pleasing color of yellow when heated.  Typical tempreture for apatite to lose color is around 500° C

Aragonite is commonly heated to around 300° C in order to crack it, readying it for acceptance of dyes.

Beryl too much data

Calcite is not suggest to be heated, unless preparing it for dyeing.

Celestite loses color when heated.

Corundum too much data

Fluorite tends to lose color at around 350° C

Galena can be melted into a liquid or oxidized into a mixed PbO powder and sulfur depending on temperature and oxygen available

Halite tends to lose color, unless it is a color caused by radium, in that case, blue halite turns violet at 340° C, turning to purple at 360° C , becoming colorless at 385° C.  These colors are apt to fade upon cooling.

Jadeite is typically treated to around 300 to convert the iron compounds in the pores to red, orange and yellow.

Kyanite does not change color upon heating to any temperature.

Langbinite turns colorless when heated.

Proustite would melt into a puddle of silver.

Pyromorphite can be melted at low temperatures but is oxidized at higher temps.

Prehnite can change color and dehydrate into a white powdery substance at high temperatures.

Quartz  too much data

Sodalite loses color when heated, but the color can come back if it is then irradiated.

Sphene/Titanite of dark hues can be heated to lighter shades, results depending on original hue and inclusions.

Spinel has a whole color range chart to be produced.

Spodumene often referred to as “hiddenite”, if it is of a Yellow, yellow-green color and is as unaffected by heat treatment as the genuine hiddenites found in North Carolina.  On exposure to light or under moderate heat of around 150° C (302° F), it takes on a strange pale olive-greenish or bluish-green hue. This bluish-green colorization of some kunzites from Brazil can be removed when heated in ordinary conventional kitchen ovens at about 150° C (302° F).  The redish-purple tints of kunzite are more permanent but tend to decolorize at 400° C (752° F).

Tanzanite coming out of the ground, does not typically have the color spectrum to be a beautiful cut stone.  It is a rare fund to find crystal with overwhelming blue/violet/red coloration, most natural zoisite has an ugly brown shade to it.  To get the brown out of the zoisite, it must be heat treated.  The color shift in zoisite is due to the inclusions of vanadium.  Heat treating the crystals makes the vanadium shift, too hot or too long in the heating chamber will cause the crystal to turn white and crumble apart.  The chart below shows the color shift due to heating and time.

overheated Tanzanite Crystal
This former gem crystal of Tanzanite was heated for too long, causing it to turn into a powdery white dehydrated mass.

Topaz – too much

Tourmaline – too much

Zeolites Lose water of hydration at elevated temperatures, crumbling into white powder. When rehydrated, some varieties produce very exothermic reactions.