Pitch for potting transformers

[GrimJosef]

Some vintage transformers and chokes, including the ones used in Quad II amps, were either coated with or even potted in pitch - the familiar black tarry substance. I have a few transformers which have lost some of this and I'd like to replace it. If you like, I want my parts to "look the part"

Can anyone tell me if there's anything special about transformer pitch as opposed, say, to the sort they use on flat roofs (which might perhaps melt at a higher temperature ?). Are there any tricks for pouring it over the transformers, or into their cans in the case of potting ? Do you know of a source of supply ?

Cheers,

GJ

[kalee20]

Well, I've not potted or enveloped a transformer in pitch. But, I'd suggest that pitch for the transformer industry may have been 'special', as opposed to just any old pitch from the roofing industry - maybe extra refining to remove any acid content etc.

Pitch does flow even at room temperatures, as it's really an extremely high viscosity liquid (there's an experiment somewhere that has pitch running through a funnel - rate is 1 drop per 30 years I seem to remember). So, if potting a transformer, be sure that it is generally kept upright, especially when powered (and hence warm).

Pitch does have the advantage that it is slightly compliant and can absorb thermal expansion and contraction of whatever it's encapsulating. Though, it is also prone to cracking at low temperatures.

Before you go looking for pitch, make sure your transformer really is a pitch one - some epoxy potting compounds look just the same. It's easy to tell , because the epoxy won't melt with a soldering iron whereas pitch will.

For the coating, assuming you have a transformer already coated in (chipped) pitch, I'd bake it in an oven for some hours (over a drip tray) at just over 100 deg C, to dry out any moisture which may have seeped in through cracks and allow the (dirty?) surfacde layes to run off. Then, I'd immerse the hot unit in molten pitch, remove, and allow to cool. When the outside starts to solidify, I'd then give it a quick dip, allow to cool, etc until you've increased the thickness to what you want - like building up a candle. Doing this will allow each new layer to adhere to the previous layer. On the other hand, if it's potted in a metal box, it's just a question of putting the baked (and still hot) transformer back in its potting box, filling with pitch, and allowing to cool.

Bear in mind that hot pitch is flammable - you should be prepared with a CO2 extinguisher handy!

[Sean Williams]

I would use synthetic potting compound if you are thinking quad 2 transformers - Molten pitch is a bad thing - too hot and you have a serious fire on your hands.

Sean

[Oldtimer74]

I would agree. It needs to be refined to remove any acid and prevent damage to the component. I would definitely not using roofing pitch. In any case these days it may not be pitch but some modern chemical substitute which has better properties for that job. Similar remarks apply to the wax coating of components. The wax has to be highly refined for the same reason. Neither can be refined at home.
Sorry I don`t know where you can buy either. I did get some wax many years ago from a manufacturer of high voltage equipment through a personal contact. I keep any pitch I can from faulty components of a reputable manufacturer.

Pat G3IKR

[GeorgSc]

Pitch is the black residue when heating pine rosin (=colophonium)
or from pine tar distillation.
The distinction between rosin and pitch was often not precise.
I doubt that there is any production of pitch today, because
beer kegs are not made from wood any more.

The black residue from coal tar formerly, today from crude oil
distillation is named bitumen.
It is used for streets, roofing and so on, there are different
qualities with respect to softening temerature.

Which grade of bitumen was used to fill transformers or capacitors,
I dont know. I would not do it, because it is a messy work,
and because it might come out on warming of the transformer.

All other drawbacks mentioned above are wrong, there never was
a kind of "purification" of bitumen with respect to acid or similar.
The only processing is to heat the vacuum residue of certain crudes
to 300°C for up to 3 days, while air bubbles percolate the hot bitumen.
This very brute treatment removes the last traces of volatiles
and oxidizes/hardenes the bitumen. (The acid content will rise!)
The degree of this treatment, and sometimes some natural asphalt
(e.g. from trinidad) blended to it, makes the bitumen "grades".

Wax:
Very problematic word! There is bees wax, some dozen sorts of
earth waxes, paraffin wax, and a hand full of synthetical waxes
(ethylene with some acrylic acid polymerized).
I guess here paraffin wax was meant. Again all ideas about
grades of different purity are wrong. The kind of process to isolate
it in petroleum refinerys makes it pure anyway.
Depending on the origin of the crude, the paraffin wax can be a
little bit softer or harder, that is all.
Concerning its use as a isolator, the wax from any (exept beeswax)
candle is o.k.
Due to a tendency to have crackles within the wax on solidifying,
wax is not as good as mineral oil as a high voltage isolator.
Regards
Georg

[Oldtimer74]

I think we shall have to agree to differ.
Pat

[Tim]

Black silicone sealant? Will dry to a black gloss.....................

[AlanBeckett]

Back in the '60s, when I worked for a TV manufacturer, we vacuum impregnated transformers in (pale) wax. It was done in a couple of large autoclaves in batches. We used to slip foreigners in with the production batches. The LOPTs on the other hand were coated/dipped in black wax. Unfortunately I haven't a clue what was actually used. All I can say is that they both melted at a fairly low temperature. I remember one of the workers splashing the black variety on to her hands and it didn't do much damage. Of course, in those days, 'Elf and Safety' wasn't so rampant!
Alan

[HG MICKE]

The wax that I use for potting or re potting Audio transformers is Microcrystalline wax,This has a higher melting point than Paraffin wax.
The advantage of wax against epoxy potting compounds is that it can easily be removed if any problem arrises,Silicone should not be used as it contains acetic acid and has shown to eat through copper.

Colin

[kalee20]

Wax for impregnating transformers is definitely not like candle wax, it is soft yet melts at a high temperature. I have seen the stuff in slab form.

As for silicones, Colin Hg Micke is partly right. Some types of single-component RTV silicone rubber do emit acetic acid during curing (bath sealant type stuff - smelling vinegary). Others cure by emitting water vapour (neutral curing grades). For electronic use, the acetoxy cure stuff is bad news, but the neutral grades are fine (and adhere very well to clean surfaces). And, there are also 2-component silicone rubber compounds, which polymerise with no vapours produced at all. But, they don't adhere particularly well so may not be good for repairing chips. Have a look at the Dow Corning web site.

[Tim]

but the neutral grades are fine (and adhere very well to clean surfaces)

Which I should have made clear. Once again I had to rush off!

[GrimJosef]

Thanks for all the replies folks. I do know that there are modern replacements for pitch (epoxies, silicones etc) and also that wax has been an alternative for years. But I'm undertaking a restoration job so I want to use (as near as I can) the original stuff. If I replaced it with something better that would be an upgrade, not a restoration.

Incidentally I am sure it's pitch (or bitumen, or tar - I recognise that there are differences between these but they're subtle as far as I'm concerned). It has that very distinctive "hot tar" smell when it melts.

I've had a further rummage on the internet and, of course, I discovered that there was a British Standard for it. BS1858 - 1973, entitled "Bitumen-based filling compounds for electrical purposes" (there's a newer version, dated 2002, but that restricts itself to the insulation of cables). The 1973 standard discusses solubility, softening point, penetration, viscosity, pouring point, electric strength, flash point, mineral matter, acidity, "injurious sulphur", adhesion and contraction. So it sounds as though the stuff is (or at least was) pretty tightly specified and the chances of getting hold of it in small quantities are looking slim

In this case I still think my best chance might be to check out roofing pitch. Provided I don't get the synthetic stuff I think it'll be the closest thing to the original material. And it will be diluted (I imagine by at least 10 to 1) by the stuff that I recover. So with luck the acidity will not have a big effect. Furthermore the roofing stuff can't be too corrosive or it would attack the nails which hold the roofing boards down. If I'm really worried I might dip the transformer itself in pure original bitumen and build up as thick a layer of this as possible before fitting it back into the can and filling the remaining space with the mixture of old and new.

Wish me luck !

Cheers,

GJ

[dominicbeesley]

Is pitch still poored into the junctions boxes used in under-road electrical distribution wiring? (I must admit the only book I have on the subject must be at least 60 years out of date!)

Dom

[kalee20]

Originally Posted by GrimJosef

Wish me luck !

Good luck GJ!

[GeorgSc]

Incidentally I am sure it's pitch (or bitumen, or tar - I recognise that there are differences between these but they're subtle as far as I'm concerned). It has that very distinctive "hot tar" smell when it melts.

If You
would have to drink beer stored in casks coated inside with bitumen
instead of pitch, You would not think the difference is "subtle" :=)

Silicone "bath" sealants:
There are three curing types:
- acid, splitting off acetic acid
- neutral, splitting off ethyl alcohol
- basic, splitting off diethyl amine.
All three need water to cure, the water is not split off as written
here.
Because water has to diffuse into and the acid/alkohol/amine
has to diffuse outward, the process is only practical up to say
10 mm, or if there is ample time (days at least), maybe 20 mm.
Silicone compounds used to seal electric equipment uses
different crosslinking chemistry, without condensation mechanisms.
regards
Georg

[Brian R Pateman]

Originally Posted by dominicbeesley

Is pitch still poored into the junctions boxes used in under-road electrical distribution wiring? (I must admit the only book I have on the subject must be at least 60 years out of date!)

Dom

Not for a long time.

These now use a two-part cold set epoxy and have for abour 30 years.

Regards,

[XTC]

Originally Posted by GrimJosef

In this case I still think my best chance might be to check out roofing pitch. Provided I don't get the synthetic stuff I think it'll be the closest thing to the original material. And it will be diluted (I imagine by at least 10 to 1) by the stuff that I recover. So with luck the acidity will not have a big effect. Furthermore the roofing stuff can't be too corrosive or it would attack the nails which hold the roofing boards down. If I'm really worried I might dip the transformer itself in pure original bitumen and build up as thick a layer of this as possible before fitting it back into the can and filling the remaining space with the mixture of old and new.

Wish me luck !

Cheers,

GJ

There's also a special grade of pitch sold in small quantities for the final finishing of astronomical telescope mirrors. Check out telescope making suppliers. It will be refined to have properties suitable for finishing mirrors, which won't necessarily be low acidity etc needed for transformers, but I'd say it was a better bet than roofing pitch. The suppliers may have a detailed spec.

e.g.

http://www.scientificmirrors.co.uk/T...pe-making.html

but there are others I'm sure.

I'd say you were better off going for a synthetic compound as used in modern transformers. You can take the quest for authenticity too far.

I wish you luck whichever way you choose to go.

Pete.

[GrimJosef]

Originally Posted by GeorgSc

Pitch is the black residue when heating pine rosin (=colophonium)
or from pine tar distillation.

Maybe in Germany it is. But in other places the word can be used to describe substances derived from coal. See for example "ASTM D 450" which is the US government standard for "Coal-Tar Pitch used for Roofing, Dampproofing and Waterproofing". Or scroll down this page http://www.hse.gov.uk/acronym/ to CTPV where the UK Health and Safety Executive explain that the acronym stands for "Coal Tar Pitch Volatiles". Or if you follow Pete's link you'll find the mirror-grinding people call their stuff "Coal Pitch Tar"

I'm sure you're right that there was once a special substance used for sealing wooden beer barrels, and that the people who made and used it called it "pitch", and that we probably can't buy it any more. But I'm looking for a different substance, and I think that the people who make and sell that may call it "pitch" or "bitumen" or "tar". And, of course, it doesn't really matter what they call it. I just want stuff which is as near as possible to what's in my transformer cans. And this, I'm pretty sure from the smell, is coal- or oil-based

Cheers,

GJ

[Sean Williams]

Potting compound is still safer to use though, and is perfectly safe on all winding wire.

Besides if this is for Quad 2 transformers you cant see what the transformer is potted in!

[GrimJosef]

Originally Posted by Sean Williams

Potting compound is still safer to use though, and is perfectly safe on all winding wire.

Besides if this is for Quad 2 transformers you cant see what the transformer is potted in!

Better ? Probably.
Cheaper ? Probably.
Easier ? Certainly.
Safer ? Yes.

Original ? No.

This is just a hobby for me, I'm not trying to save time or money (ask my wife !). Call it perverse if you like, but my satisfaction comes from knowing that I've restored something to as near its original condition as I can get it. And these transformers were originally potted in nasty, smelly, black stuff ...

Cheers,

GJ