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"Sugar rockets" or sparklers made of metal fillings, potassium nitrate and some carbon compound are easily made, and there is a lot of tutorials and videos on the internet suggesting them as relatively safe and non toxic DIY fireworks.

However reading on the potassium nitrate reaction, it decomposes to potassium nitrite when heated. This compound is toxic, and is suspected to be mutagenic as well. It is also an oxidizer on its own.

So I have safety concerns and questions:

  1. Do the charred remains of potassium nitrate fire experiments need to be handled as toxic waste?
  2. If potassium nitrite is an oxidizer, what does it decompose to when burned along with a carbon compound?
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Potassium nitrate is transformed into potassium nitrite only when heated alone. And it is not really toxic, as it is added in ham to prevent its too early oxidation in air. When heated with charcoal, sulfur or any other oxydable substance, potassium nitrate is transformed into $\ce{K2CO3}$ or $\ce{K2S}$. For example, black powder is a mixture $\ce{KNO3 + C + S}$ that reacts violently according to ; $$\ce{2KNO3 + 3 C + S -> K2S + 3 CO2 + N2}$$ And if the explosion is made in contact with air, the hot $\ce{K2S}$ is quickly oxidized into sulfate according to $\ce{K2S + 2 O2 -> K2SO4}$

Sugar rockets are made of ordinary sugar $\ce{C_{12}H_{22}O_{11}}$ and $\ce{KNO3}$. This mixture reacts according to the two following equations : $$\ce{C_{12}H_{22}O_{11} -> 12 C + 11 H2O}$$ $$\ce{4 KNO3 + 5 C -> 2 K2CO3 + 2 N2 + 3 CO2}$$

So the ashes of any combustion reaction are mostly made of $\ce{K2CO3}$ which was the main constituant of the wood ashes. These ashes was used in the Middle Age for washing clothes by our grand-grand-mothers. They are not considered as toxic

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