Aluminium in homemade liquid castile soap?
The question is actually is skin absorption of Al3+ a health risk. The answer from a legal perspective varies depending on where you live. In the USA, for example, the answer is no, however, there appears to be a growth industry in selling Aluminum-free products, but not in Europe, where apparently stick deodorants containing the Aluminum ions are banned.
But what is the suspected chemistry possibly suggesting health problems with the Aluminum ion? My prior exploration of this topic points to some recent research. Based upon a review of two articles noting the pro-oxidant property of the aluminum ion, a potential health risk pose by the ability of aqueous Al3+ to recycle transition metals (like Fe, Cu, Mn,..) to lower valence states (via the superoxide radical anion) in Fenton and Fenton-like reactions, contributing to so-called oxidative stress related diseases (including Alzheimer's disease, various cancers,...). More explicitly, with iron ions, for example, recycling between ferric and ferrous ions feeding a fenton reaction via superoxide radical anion (produced in our bodies):
$\ce{ Fe^{3+} + O2^{•−} &lt;=&gt; Fe^{2+} + O2}$
Source: See Eq (1.xxxiii) which can feed a fenton reaction forming the hydroxyl radical:
$\ce{ Fe^{2+} + H2O2  -&gt; Fe^{3+} + •OH + OH-}$
The first alluded to article is Oxidative Stress Gated by Fenton and Haber Weiss Reactions and Its Association with Alzheimer’s Disease by Tushar Kanti Das, et al, published in Archives of  Neuroscience, July 2014 2(3): e20078, DOI: 10.5812/archneurosci.20078.
The authors cite, in Figure 4, “Formation of Aluminum Superoxide Semi reduced Radical Ion and Aluminum Superoxide Complex (43)”, with described reactions proceeding as follows (also adopting notation and water complexing from the second article):
$\ce{ [Al(H2O)4]^{3+} + O2^{•−}   &lt;=&gt; [Al(O2^{•−})(H2O)4]^{3+} }$
$\ce{ [Al(O2^{•−})(H2O)4]^{3+} + Fe^{3+} -&gt; O2 + [Al(H2O)4]^{3+}  + Fe^{2+} }$
And, in the presence of H+ :
$\ce{ [Al(H2O)4]^{3+} + O2^{•−} + H+  &lt;-&gt; [Al(O2^{•−})(H+)H2O)4]^{3+} }$
$\ce{ [Al(O2^{•−})(H+)H2O)4]^{3+} + [Al(O2^{•−})(H+)H2O)4]^{3+} -&gt; 2 [Al(H2O)4]^{3+} + H2O2 + O2 }$
Note per below, the action of hydrogen peroxide on ferric also leads to ferrous:
$\ce{ H2O2 &lt;=&gt; H+ + HO2- }$
$\ce{ Fe^{3+} + HO2- -&gt; Fe^{2+} + HO2• }$
or, pH&gt; 4.88, we have:
$\ce{ H+ + O2^{•−} -&gt; HO2•}$
Another work: Pro-oxidant Activity of Aluminum: Stabilization of the Aluminum Superoxide Radical Ion by J. I. Mujika, F. Ruiperez, I. Infante, J. M. Ugalde, C. Exley, and X. Lopez in J., published in Phys. Chem. A 2011, 115, 6717–6723, American Chemical Society, dx.doi.org/10.1021/jp203290b .
In the Mujika article to quote “In addition, the presence of LMM ligands such as citrate could also have an indirect effect in the oxidation capacity of aluminum by augmenting the bioavailability of Al3+ species, shifting the formation of Al(OH)4- to higher pH’s. However, one should also take into account the effect of citrate chelation itself in the thermodynamic equilibrium of [AlO2•]2+ formation.”
So, my short takeaway, our body produces the short-lived superoxide radical anion capable of recycling problematic transition metals which can engage in fenton (and fenton-like) reactions forming powerful radicals. The latter can attack your DNA leading to associated diseases. The role of Al3+ is that it can form a complex with the superoxide, likely increasing its effective half-life and augmenting destructive radical activity in one's body.
To answer the question: &quot;Is the soap safe to use?&quot;, likely that it is a personal decision based on one's understanding of the proposed underlying chemistry, as jurisdictions apparently vary in opinions.