Discharge technique is the intentional removal of dye from a fabric or fibre, preferably without damaging the fabric or fibre.  You can use simple household bleach - but the results can be very unpredictable, you have to neutralise the chlorine otherwise it will continue to react and will damage your fibre, and of course, should never be used on protein fibres...  The alternative is to use one of a group of chemicals that ONLY removes the dye, without damaging the fibre.  

TUD     -  thio urea dioxide  - discharges Drimarene K and Procion MX dyes.     500g  $15.00     100g  $5.00

Formosul     -  sodium formaldehyde sulphoxylate  -  discharges acid dyes.     500g  $20.00

Sodium Hydrosulphite     -  1 kg  $21.00     250g  $10.50

Associated chemicals...

DR 33     -  for thickening discharge agent so you can then print with it     100g  $7.00     250g  $14.00

Soda ash     -  mild alkali     500g  $5.25

Acetic acid 30%     -  250ml   $10.00

Anti Chlor     -  chlorine neutraliser     100g  $7.00

Discharge sampler kit  -  all 3 discharge agents plus instructions.  Enough of each to discharge 100g of fabric     $15.00

Drimarene K discharge kit  -  TUD, DR-33, stencil, gloves, fabric sample & instructions.                                               Enough to make up 250ml of paste      $10.00

TUD (thio urea dioxide) works best on fibre reactives and direct dyes eg. Drimarene K and Procion, and Solar and Star dyes.  Formosul works best on acid dyes eg. Sandolan and Lanasyn dyes.  Sodium Hydrosulphite works best on direct dyes eg. Solar and Star.

If you have dyed the fabric in question yourself you will naturally know which dye you have used and so be able to match the discharge agent.  If you are trying to remove the colour from commercial fabric it can be a hit and miss affair...  acid dyes are used on silk and wool, but will not dye cotton or the other plant based fibres - so Formosul would be pointless on cotton etc.  But silk can also be dyed with fibre reactives....  so the choice is wide open there !
And these products come with no guarantee they will discharge the dye.  Some colours will not discharge at all - even when you have what should be the correct discharge agent. And not all dyes will discharge back to white.  Some will go to a paler version, some go to an interesting other colour... 
 

INSTRUCTIONS

TUD
Immersion recipe
For every 100g of fabric you will need:
TUD 1/4 teaspoon
Soda ash (sodium carbonate) 1 teaspoon
Water  2 litres
1. wet out fabric
2. dissolve soda ash in 100mls hot water
3. add water to make up to 2 litres, add fabric, bring to boil
4. when boiling, add the TUD slowly, stirring.  Do not add all at once !
5. simmer for 20 minutes, stirring gently
6. wash in warm soapy water.  Rinse.

FORMOSUL
Immersion recipe
For every 100g of fabric you will need:
Formosul  10g
Acetic acid 30%   20ml
Hot water  2 litres
1. wet out fabric
2. dissolve Formosul in the hot water, stir
3. carefully add the acetic acid, stir
4. add the fabric
5. raise the temperature to boil.  Keep at a simmer.  Stir intermittently for 30- 40 mintes
6. drain then rinse well in hot water
7. wash in warm soapy water.  Rinse.

NB. Not all acid dyes can be stripped esp those in the blue and green range.

SODIUM HYDROSULPHITE
Immersion recipe
For every 100g of fabric you will need:
Sodium hydrosulphite 3g
Caustic soda  2g
Water 1 litre
1. wet out fabric
2. dissolve the sodium hydros. and caustic soda in the water
3. add the fabric
4. heat to 90° C (not quite the boil) over 15 minutes, stirring occasionally
5. hold there for 45 minutes, again stirring
6. rinse in warm water, then cold.
7. wash in warm soapy water.  Rinse.