Is my maths/chemistry correct?

[nframe]

I have some rebatched soaps that are perfectly OK except that they do not look particularly nice. I thought of using them to make laundry/dish soap since I don't like to idea of throwing them away. But as they have a superfat of 5%, I thought of the following:

First of all, I grate the rebatched soap very finely. Then, I make a new batch with an SF of -10% and add the grated rebatched soap to it. My question is: will the lye "eat up" the 5% SF of the rebatched soap and will I end up with a soap of 0% SF?

It sounds logical to me but will it work?

 

[The Efficacious Gentleman]

I think you'll have to melt the old soap in to the new to make sure. Plus, the you only need a -5% SF if that batch size is the same as the two +5% batches combined.

So if you have the two original batches, 1 at 500g and the other at 250g, both with 5% SF, you need to make a 750g batch with -5%SF and combine it all to have a 1,500g batch with 0% SF.

Of course, I think that it might be possible to work out how much raw lye solution to mix with the two original batches in a rebatch to get them down to 0% SF rather than making a huge batch, but I can't get my head around it at the moment.

ETA: I have an idea -

Take your original recipe, put it in to the calc. This is what you did make.
Change the SF to 0% and calculate
Compare the two lye amounts and add that extra lye in with the water for the rebatch.

Now, that is just theory. I would feel better if wiser minds than mine give it a thought and feedback

 

[Obsidian]

I've rebatched soap and added extra lye water to bring the SF down. It worked fine but make sure your soap is completely melted before adding in the lye.

 

[nframe]

Thank you. I had not thought of melting the old soap first. That makes sense.

 

[Nevada]

I have some rebatched soaps that are perfectly OK except that they do not look particularly nice. I thought of using them to make laundry/dish soap since I don't like to idea of throwing them away. But as they have a superfat of 5%, I thought of the following:

First of all, I grate the rebatched soap very finely. Then, I make a new batch with an SF of -10% and add the grated rebatched soap to it. My question is: will the lye "eat up" the 5% SF of the rebatched soap and will I end up with a soap of 0% SF?

It sounds logical to me but will it work?

Add washing soda (sodium carbonate) to the grated 5% Soap. Washing soda is mildly alkaline.

 

[The Efficacious Gentleman]

Add washing soda (sodium carbonate) to the grated 5% Soap. Washing soda is mildly alkaline.

But would it react with the oils? It's not the acidy that would be the issue, but rather the free oils still sitting on the clothes.