Page 2 - Sri Lankan Traditional methods and instruments of Geuda Heat Treatment

1. Introduction

Sri Lanka is one of the famous countries for geuda gemstone which has the potential to convert into
premium blue colour through heat treatment. There are several types of geuda varieties and their
subcategories (for example Diesel geuda could be subdivided into young, normal thick, very thick
depending on its appearance under transmitted light). Table 01 shows different types of geuda
varieties and their optimum temperatures and environmental conditions for blue colour
enhancement.

Table 01: Optimum temperatures and environmental conditions for different types of geuda

o
Type of Geuda Heating Temperature ( C) Condition Colour after the heat
treatment
Ottu 1600 – 1650 Reducing Blue
Dum Geuda 1600 – 1650
Diesel Geuda 1750 – 1800
Milky Geuda 1750 – 1800
Silky Geuda 1800 – 1900
Deguna Stones 1050 – 1500 Oxidizing Red
Kowangu Pushparaga 1800 – 1900 Yellow

Sri Lankan gem traders had used various types of furnaces and instruments for the geuda heat
treatment from the end of the nineteens with the knowledge of heat treatment. In the past furnaces
were very simple in structure and with the research and development process nowadays even
programmable electric furnaces are available for geuda heat treatment. The article endeavors to
discuss different types of furnaces and equipment for geuda heat treatment. At the early stages,
there were set of furnaces. According to the (Munasinghe, 1993),Thai gas furnace, Toda Furnace and
Condor furnace used over the other furnaces in Sri Lanka before Lakmini gas furnace was
manufactured.

Table 02:Various types of furnaces (Source: (Themelis, 1992))
o
Type of Furnace Type of Fuel Source of Thermal Maximum Temperature( C)
Energy
Open-Flame Gas Gas + Air 1600 – 1700
Oxygen + LPG 1700 – 1750+
Oxygen + LPG + Air 1800 – 1850+
Acetylene + Oxygen 2800 +
Solid Coke + Air 1000 – 1200
Lignite + Air 1400 – 1600
Charcoal + Air 900 – 100
Liquid Diesel + Air 1600 – 1700
Diesel + Air + Oxygen 1750 – 1800+
Electric Resistance Molybdenum Disilicide 1700
Graphite 2250
Other RF (radio frequency) 1900+
Induction