What is the name of that which is a close match for the density of ice, which encouraged its widespread use in the 1990s?
Drilling fluids are necessary for borehole stability in deep cores, and can also be used to circulate cuttings away from the bit.  Fluids used include water, ethanol/water and water/ethylene glycol mixtures, petroleum fuels, non-aromatic hydrocarbons, and n-butyl acetate.
Water is the cheapest and cleanest option; it may be present on the glacial surface or may be created by thermal drilling.  In cold ice some form of antifreeze is necessary, or heat must be reapplied by reaming the hole periodically.
Ethanol and water.  Ethanol acts as an anti-freeze in water; at sufficient concentrations it can reduce the freezing temperature of the mixture to well below any temperature likely to be encountered in ice drilling.  The concentration must be chosen to prevent the liquid freezing and also to maintain the borehole against the ice overburden pressure.  Because the density of the mixture decreases with lower temperatures, vertical convection will develop in boreholes where temperatures decrease with depth, as the lighter mixture rises.  This causes slush to form in the borehole, though successful drilling is still possible.  Ethanol is one of the cheapest options for a drilling fluid, and requires less storage space than other options because in use it is diluted with water.  A Soviet expedition left an 800 m borehole in Antarctica filled with ethanol and water at an ice temperature of −53 °C; after 11 months the borehole remained open and drilling was resumed with no problems.  A problem with this option is that the mixture will penetrate cores that have cracks.
Ethylene glycol and water was used at Camp Century in 1966 in the lower part of the hole to dissolve the cuttings.
Petroleum fuels.  This includes diesel, jet fuel, and kerosene. They are inexpensive and easily available, and were once in common use; disadvantages include flammability and the aromatics they contain, which are a health hazard.
Non-aromatic hydrocarbons.  As of 2009 these had become the most commonly used drilling fluids; eliminating the aromatics resolved the health issues with these fluids.  They are significantly more expensive than untreated petroleum fuels.
n-Butyl acetate.  A widely used fuel in the 1990s, because it is a close match for the density of ice, this is now unpopular because it dissolves many materials, which prevents their use in the drilling equipment it comes in contact with.  It is also flammable and corrosive, and protective clothing and in some cases masks may be necessary for people exposed to it.
n-Butyl acetate