I think this is the one of the best ways to unblock a seriously clogged toilet, especially when you have disposed of organic waste into it and have tried a plunger, wire hanger, dishwashing liquids, loads of water, etc., and have been unsuccessful.
Please take extreme care while handling caustic soda. You will require about 3 kgs of caustic soda, a plastic bucket, a plastic pipe or any long plastic object to help stir, rubber/plastic gloves, and goggles for your own safety and protection.
It is ideal to do this at night. Pour about 3 to 4 liters of water in a plastic bucket, cover your nose with a piece of cloth, and slowly add about 1 kg of caustic soda into the water -- very slowly. Stir the mixture slowly, also. Make sure the room is well ventilated and do not inhale the fumes. Get away from the room if it is taking longer to mix the water and caustic soda. Always add the caustic soda to water; do not do it the other way. Do not add water to caustic soda. The exothermic reaction between water and caustic soda will make the water boil quickly, so adding water directly to caustic soda is not advised.
The water will heat up during mixing and will generate fumes. Do not touch the mixture with you bare hands as it is highly corrosive. If a drop of the mixture falls on your skin, wash it with lots of water.
Once the solution is mixed, slowly pour it into the empty toilet bowl, cover the toilet seat and get away from the toilet for about 45 minutes. Once again, mix the caustic soda and water as described above for a second time. Pour it slowly into the toilet and leave it overnight.
In the morning, pour a couple of buckets of steaming hot water into the toilet from about waist height. By now the toilet should flush out quickly. If not, mix water and the caustic soda again as described above and pour it into the empty toilet. Wait for a couple of hours and pour more hot water into the toilet.
Never pour caustic soda directly into the toilet. Caustic soda crystals can solidify inside the toilet pipes, creating a bigger block. So it is advised to mix the caustic soda in water first and use the solution to unblock your clogged toilet.