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So recently II have been making a few times some slow burning fuses with: 34 g potassium nitrate, 26 g sugar, 60 g of water and some wool yarn. However when I light it up it burn a few seconds and just lights off.

I have also dried it at 130 °C for 20 min. How can I make it last the length of the yarn?

edit: I was using cotton and not wool (sorry).

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  • $\begingroup$ @Poutnik i have tried cotton yarn and some thick cardboard like yarn which were even worst $\endgroup$
    – goAT2160
    Commented Apr 10, 2023 at 8:20
  • $\begingroup$ Why adding water ? $\endgroup$
    – Maurice
    Commented Apr 10, 2023 at 8:46
  • $\begingroup$ @Maurice I assume soaking KNO3 + sugar solution to wool thread + drying. $\endgroup$
    – Poutnik
    Commented Apr 10, 2023 at 9:08

1 Answer 1

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Have you evaluated the ideal theoretical or experimental ratio $\ce{KNO3}$ : sum of sugar and cotton?


Stoichiometric mass ratio $\ce{KNO3}$: sucrose is 3 : 1.

$$\ce{48 KNO3 + 5 C12H22O11 -> \\ 60 CO2 + 55 H2O + 24 N2 + 24 K2O}$$

(A part of potassium oxide and carbon dioxide may end as potassium carbonate, but it does not affect the ratio.)

The needed ratio is even higher as the cotton thread is being burnt as well. Your ratio is way too low. You may need to use hot solution, as $\ce{KNO3}$ solubility dramatically raises with temperature.


As info has been corrected, the stoichiometric ratio is for cotton similar as for sugar/sucrose, i.e. 3 : 1.

You can estimate the overall mass ratio from the nitrate/sugar ratio in solution and final/initial thread mass ratio.

If use use nitrate without sugar, the final thread mass should be 4 times bigger than its initial mass.

If you do use sugar and if the solution nitrate/sugar mass ratio is $a$ and the final/initial thread mass ratio is $b$, then the total nitrate/(sugar+cotton) ratio is

$$\frac{(b-1)a/(a+1)}{1+(b-1)1/(a+1)} = \\ \frac{(b-1)a}{a+b}$$

If we respect the desired 3 : 1 ratio and if we use the solution ratio $a$, then the final/initial tread mass ratio must be:

$$3 = \frac{(b-1)a}{a+b}$$

$$4a + 3b = ab$$

$$b=\frac{4a}{a-3}$$

I think it is worthy to try:

  • hot saturated $\ce{KNO3}$ solution, without sugar
  • $\ce{KNO3}$/sugar ratio 4-6/1, as correction on cotton.
  • as supporting procedure, accumulative deposion by repetitive soaking/drying.
  • to let (repeatedly) $\ce{KNO3}$ crystallize on the thread in saturated solution being cooled down.

(The original wool part had been left for completeness, but is not relevant any more.)

Estimated stoichiometric mass ratio $\ce{KNO3}$ : wool is 4.2 : 1. (based on typical ranges of major elemental wool composition, represented by the model empirical formula $\ce{C15O5N4H20}$ ).

$$\ce{14 KNO3 + C15O5N4H20 -> \\ 15 CO2 + 10 H2O + 9 N2 + 7 K2O}$$

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