Black scale and green soft soap

Paging through some back issues of "the other magazine" (EIM), I came across a letter suggesting that a good way of preventing black scale when hardening silver steel is by dipping the item to be hardened in green soft soap prior to heating.
When the item is heated the soap first burns yellow and then congeals forming a black protective coating. Heating is then maintained until cherry red, the item is held at this temperature for a half a minute or so before plunging into a jam jar of cold water containing 2 to 3 teaspoons of salt.

Needing a D-bit over the weekend I decided to give this method a try. Not having any green soft soap, I thought perhaps that the green washing up liquid from the kitchen sink may be a suitable alternative. It didn't seem to do any harm, the tool came out as hard as expected, however it was as black if not blacker than my previous attempts at hardening sans the soap.

A subsequent enquiry for green soft soap at the local chemist had the young lady direct me to a shelf full of smelly substances in pump bottles, not too dissimilar to the "hand washing lotion" already to be found in our bathroom at home. Containing a multitude of scents and hand softening salves, it seems unlikely that this would be much use for hardening, and if anything more likely to have the opposite effect on any self respecting silver steel!

So, what is green soft soap? Is there anything special about it that makes it suitable for use when hardening silver steel? And if so, is there a generic type that is likely obtainable here in Canada?

Clive

en.wikipedia.org/wiki/Softsoap

Ordinary bar hand soap is fine. Does not have to be green.

Simple when one knows where to look, thanks

Oh excellent thanks, I'll give bar hand soap a try next time.
Clive

Wrong link I think. Try this **LINK**

It explains the different manufacture from ordinary hard soap using potassium hydroxide instead of sodium hydroxide. I've got a pot of this bought in the Netherlands about 25 years. I've just had a look- it's not green any more! It was sold mostly as a non-toxic insecticide. I suggest you try and find something like this **LINK**.

This also seems to be the real stuff http://www.ebay.co.uk/itm/POTTERY-MOULD-MAKERS-RELEASE-AGENT-SOFT-SOAP-5-KILO-/371409016229?hash=item5679b3d5a5

HTH,

Rod

Edited By Roderick Jenkins on 17/08/2015 19:53:49

Thanks Rod, That is very helpful.

From the Wikipedia link it seems that potassium rather than sodium hydroxide is the key difference, which means that the "potters mould making release soap" on eBay is likely the genuine thing.

I wonder though, whether soft soap is that much more effective than regular bar soap at preventing black scale that it is worth seeking out, or is it, being in liquid form, just more convenient to use.

Clive

Aahh ... The oh-so-subtle difference between 'Softsoap' and 'soft soap'.

MichaelG.

Borax.

I'm still a bit confused as to what to look for in the supermarket. Are the pump action handwash bottles a good idea or is there any other stuff readily available that works?

Doug

Doug,

Based on Rod's information ... In the supermarket: Don't rely on the name; read the list of 'ingredients'.

MichaelG.

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Note: Back when much of this engineering wisdom was devised:  ... 'Hard' Soap was a very different thing to what we see now as 'Bar' Soap. 

P.S. ... Just found this site, which may be useful.

Edited By Michael Gilligan on 18/08/2015 09:18:14

Edited By Michael Gilligan on 18/08/2015 09:32:20

In our bathroom any block of soap soon goes wet and soggy, this can be used for coating the steel and could be called soft soap. It seems to work OK.

Long ago I tried to follow GHT's recommendation of soft soap to prevent scaling but I have never had much success with the anonymous product I obtained from a friend answering this description. From memory it was indeed green but, years later, it has turned a dark chocolate colour.

I just turned up Tubal Cain's thoughts on the subject in WPS 1. In the absence of ceramicist's "Anti-Scale Paint", he favoured powdered chalk mixed with water or meths and, whilst aware of the horologist's practice of using soft soap, surprisingly had not tried it himself.

"Bar soap" here at home is Pears and the list of ingredients mentions a number of sodium compounds which suggests it is unsuitable if I correctly understand the Wiki disambiguation entry; the Pears helpline did not know whether it was a sodium or a potassium process. One should obviously read the label but it seems that the production process may not be given, so my question is: does the mention of sodium compounds indicate unsuitability for this purpose?

Roderick Jenkins' suggestion may be the answer but who needs 5 kilos?

ega,

Sorry ... no glib answer from me; but hopefully someone will come-up-trumps.

What I do know, is that Pears bar soap is not the product it once was. I remember that, as a child, I watched my Grandmother 'welding' the remaining sliver of Pears transparent soap into the thoughtfully-provided recess in the new bar [using just a drop of warm water] ... It doesn't work with the current product, which is made in India.

I won't bother listing all the ingredients, but the first on the list [and therefore the highest percentage] is Sorbitol.

MichaelG.

Michael Gilligan:

Thanks for your comments. When I rang the help line, in response to my saying I thought that the product was less transparent than before, they mentioned that the proportion of glycerine had been increased . Apparently, it has been made in India for twenty years which surprised me. I can still manage the welding trick by leaving the sliver on the wetted replacement overnight; it was this "soft" characteristic that made me think it might be suitable for the anti-scaling purpose.

Needless to say, the helpful woman on the help line had not heard of Punch's "Two years ago I used your Soap, since when I have used no other".

PS: punch line rather than help line!

Edited By ega on 18/08/2015 12:51:23

Soaps are salts of carboxylic acids such as stearic acid (stearine). If it is the sodium salt it is the familiar hard soap used for hand washing and if it is the potassium salt then it is soft soap which is used as an insecticide and mould release agent. Chemically they behave very similarly and both work well at preventing scale formation.

The liquid hand wash that is sold in bottles with a pump top are not usually soap but sodium lauryl sulphate which is basically the same as many washing up liquids and shampoos. It also contains other compounds such as thickening agents to give it body. When heated to red heat the sodium lauryl sulphate will be reduced to sodium sulphide which may introduce sulphur into the surface of the steel. This could cause cracking and render the steel liable to corrosion afterwards.

I have, in the past, successfully used both soft soap and hard soap for scale prevention but nowadays I use common household soap because it is much more readily available. With a knife it is easy to shave the block into slivers that can then be crushed easily to a powder. Heat the metal a little and dip it in the powder . The soap will melt and stick to the surface. Then heat the metal to high temperature for heat treating. Much of the soap drips off as the metal is heated but sufficient remains to prevent scaling. An even better alternative is to mix the powdered soap with powdered chalk (roughly 50:50). This mix does not drip off of the hot metal and remains as a crust on the surface.

Mike

Re: Pears Soap

As ever ... Wikipedia is worth a look.

MichaelG.

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That nicely demonstrates the fact that it's a different recipe.

With the original, it was just a matter of wetting both surfaces and then weld them together with finger-pressure applied for a few seconds.

MichaelG.

I must try this 'soap' stuff some time.

Neil

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No ... cultivate the grimy look

Makes you look like a proper engineer.

MichaelG.