Converting a glaze recipe from PARTS to WEIGHT
When potters see a glaze that is listed in PARTS it is often confusing how to convert it to WEIGHT( In the potter's standard method of weighing - grams).
Earlier potters didn't have digital scales or even scales for that matter so they often listed glazes by scoop of a material. Like 4 scoops of feldspar /3 scoops of silica/2 scoops of whiting/ and 1 scoop of kaolin. This became the 4/3/2/1 recipe. Which was traditionally listed as parts not weight. (This story may or may not be true but shows how potters with no weighing method could quickly make a glaze using the part recipe method.)
The part recipe method may have worked in the old days and may even have appeared to be reproducible since many glazes have a wide range of workabliity. But it certainly is not an accurate way to get the same recipe every time you mix because the weight of the material in the scoop is dependent on whether the scoop is packed tightly or loosely. Which means that every scoop could be a different weight.
This is analogous to professional bakers who list bread or pastries recipes by weight not by volume (like one cup or a 1/4 cup). The reason is that, a cup of "flour" varies in weight by the type of flour you use, whole wheat, cake, pastry, rye, etc. And the amount of flour you get in a cup / scoop of flour varies greatly on whether it is sifted, dipped, spooned in or packed. So professional bakers use a weight method because they are trying to insure the accuracy of measurements.
http://www.joyofbaking.com/WeightvsVolumeMeasurement.html
Another confusing thing about baking is that there is a cup by fluid and cup by weight which are two entirely different things. That is why you have a liquid cup measure (8 ounces) and a measuring cup for dry materials. (The only reason I know this is that I used a liquid cup measure to add flour to a bread recipe and it came out poorly.)
But I digress, back to ceramic glazing. If we want accuracy and reproducible results with parts recipes we will have to convert them to weight (or grams).
Often people will do this by taking a recipe and simply retotaling it.
Like:
4 Nepheline Syenite
2 Silica
1 Kaolin
_________
7 parts Total
Then divide each ingredient by the total, 7, and multiply by 100 to get the retotaled percent of each ingredient.
4 Nepheline syenite divided by 7 times 100 = 57.14
2 Silica divided by 7 times 100 = 28.57
1 Kaolin divided by 7 times 100 = 14.29
Percent of Parts
57.14% Nepheline Syenite
28.57% Silica
14.29% Kaolin
But this isn't grams it is merely the percent of parts in the recipe.
But don't take my word for it, let's test it by taking an actual scoop (part) and weighing it ...(I used 1/3 cup scoop)
I get :
Earlier potters didn't have digital scales or even scales for that matter so they often listed glazes by scoop of a material. Like 4 scoops of feldspar /3 scoops of silica/2 scoops of whiting/ and 1 scoop of kaolin. This became the 4/3/2/1 recipe. Which was traditionally listed as parts not weight. (This story may or may not be true but shows how potters with no weighing method could quickly make a glaze using the part recipe method.)
The part recipe method may have worked in the old days and may even have appeared to be reproducible since many glazes have a wide range of workabliity. But it certainly is not an accurate way to get the same recipe every time you mix because the weight of the material in the scoop is dependent on whether the scoop is packed tightly or loosely. Which means that every scoop could be a different weight.
This is analogous to professional bakers who list bread or pastries recipes by weight not by volume (like one cup or a 1/4 cup). The reason is that, a cup of "flour" varies in weight by the type of flour you use, whole wheat, cake, pastry, rye, etc. And the amount of flour you get in a cup / scoop of flour varies greatly on whether it is sifted, dipped, spooned in or packed. So professional bakers use a weight method because they are trying to insure the accuracy of measurements.
http://www.joyofbaking.com/WeightvsVolumeMeasurement.html
Another confusing thing about baking is that there is a cup by fluid and cup by weight which are two entirely different things. That is why you have a liquid cup measure (8 ounces) and a measuring cup for dry materials. (The only reason I know this is that I used a liquid cup measure to add flour to a bread recipe and it came out poorly.)
But I digress, back to ceramic glazing. If we want accuracy and reproducible results with parts recipes we will have to convert them to weight (or grams).
Often people will do this by taking a recipe and simply retotaling it.
Like:
4 Nepheline Syenite
2 Silica
1 Kaolin
_________
7 parts Total
Then divide each ingredient by the total, 7, and multiply by 100 to get the retotaled percent of each ingredient.
4 Nepheline syenite divided by 7 times 100 = 57.14
2 Silica divided by 7 times 100 = 28.57
1 Kaolin divided by 7 times 100 = 14.29
Percent of Parts
57.14% Nepheline Syenite
28.57% Silica
14.29% Kaolin
But this isn't grams it is merely the percent of parts in the recipe.
But don't take my word for it, let's test it by taking an actual scoop (part) and weighing it ...(I used 1/3 cup scoop)
I get :
RANDOM SCOOP WEIGHT:
79.3 grams Nepheline Syenite
84.8 grams Silica
53.7 grams Kaolin
That is a serious discrepancy in how much a PART actually WEIGHS.
I would suggest that any potter who wants accuracy and repeatably should weigh out ingredients rather than use parts.
Now I will say that if you are consistent with your scoops you will get pretty close to the same amount every time that you scoop, but to share that information with others is hard. Should they fluff the scoop or pack it or just randomly scoop it?
And if you are making a recipe and you just added a new bag of EPK to the bin and then grab a scoop as usual, it will be a fluffed up scoop rather than a random scoop you may have been used to. So even your scoops will be inaccurate.
Now to take the random scoop recipe above and convert it to weight I get:
79.3 grams of Nepheline Syenite
84.8 grams of Silica
53.7 grams of EPK in each scoop.
Then I mulitply by 4 parts Nepheline Syenite and 2 parts of Silica and 1 part of Kaolin=
79.3 grams Nepheline Syenite
84.8 grams Silica
53.7 grams Kaolin
PACKED SCOOP WEIGHT:
96.4 grams Nepheline Syenite
105.6 grams Silica
66.8 grams Kaolin
66.8 grams Kaolin
FLUFFED UP SCOOP WEIGHT:
68.6 grams Nepheline Syenite
78.2 grams Silica
47.7 grams Kaolin
47.7 grams Kaolin
So you see it is quite a variation . The range in the weight of a scoop is from :
68.6- 96.4 grams Nepheline Syenite
47.7- 66.8 grams Kaolin
78.2- 105.6 grams Silica
About a 20 -30% variance.
About a 20 -30% variance.
Now I will say that if you are consistent with your scoops you will get pretty close to the same amount every time that you scoop, but to share that information with others is hard. Should they fluff the scoop or pack it or just randomly scoop it?
And if you are making a recipe and you just added a new bag of EPK to the bin and then grab a scoop as usual, it will be a fluffed up scoop rather than a random scoop you may have been used to. So even your scoops will be inaccurate.
Now to take the random scoop recipe above and convert it to weight I get:
79.3 grams of Nepheline Syenite
84.8 grams of Silica
53.7 grams of EPK in each scoop.
Then I mulitply by 4 parts Nepheline Syenite and 2 parts of Silica and 1 part of Kaolin=
317.2 Nepheline Syenite
169.6 Silica
53.7 Kaolin
----------------
540.5 total
then I retotal by dividing each amount by the total times 100-
58.68% Nepheline Syenite
31.38% Silica
9.94% Kaolin
So you see this is fairly different that the PERCENT OF PARTS (ABOVE)
57.14 Nepheline Syenite
28.57 Silica
14.00 Kaolin
It may be close but that depends on the weight of the different materials you use and the amount of parts you use so some recipes will be off even more.
169.6 Silica
53.7 Kaolin
----------------
540.5 total
then I retotal by dividing each amount by the total times 100-
58.68% Nepheline Syenite
31.38% Silica
9.94% Kaolin
So you see this is fairly different that the PERCENT OF PARTS (ABOVE)
57.14 Nepheline Syenite
28.57 Silica
14.00 Kaolin
It may be close but that depends on the weight of the different materials you use and the amount of parts you use so some recipes will be off even more.
oh...thank you so much...calculating for glazes is a bit daunting.
ReplyDeleteHad to fix it Indu.
ReplyDeleteHi John, I was sure I wrote a comment but here we go. The example in CM is not listed as gram weight and is indeed percent weight. You are very helpful in showing how a part must be dumped out and weighed in order for it to be gram weight. Each part will have a different gram weight depending on how it is packed. One would have to be vigilant in packing a part, to get the same part weight. But parts are not meant to be as precise as gram recipes. As you also pointed out, if a recipe has large parts or percents of fewer, single ingredients, chances are it will be similar to the glaze measured in grams. The more ingredients, smaller measurements will result in varied results. People who meaure in parts do not want to measure in grams! But for accuracy and reliabilty to reproduce a glaze, gram weight is the way. All very helpful, thank you.
ReplyDeleteI totally agree Alisa and thanks for writing the article. We need good writers and articles in Ceramics Monthly! You did a great job.
ReplyDeleteThe interesting thing when I ran the test for what a part weighted, even though they varied greatly across a type ... if I just did them all as one type like packed or loose, then they were withing a gram or two which was very surprising !
Sorry it took so long to post this Alisa..been busy.
ReplyDeleteOk, I've read this several times and I'm still confused. Are you saying that I have to take every ingredient of every recipe and run it through this math? Isn't there a simpler way to convert this?
ReplyDelete